SIST ES 202 975 V3.1.1:2026

• Abstract:

  The present document specifies requirements for relay services, covering both the relay service and its invocation over ICT networks. The present document applies to all types of relay services that enable users with functional limitations related to hearing, vision, speech or cognition, to communicate in real time with other users of electronic communications services. The present document applies to, inter alia, text relay services, speech-to-speech relay services, video relay services, captioned telephony services, lipspeaking relay services and cognitive and memory support services. Functional requirements for the invocation of relay services within communication platforms are included. Implementation of the invocation functions is needed to ensure equal access to electronic communications services. Included use cases cover emergency communications, communication with other individual users, and the integration of relay services in multiparty conferencing scenarios. Requirements are specified for services provided on a 24/7 basis, as well as for limited-hour services.

• Status: Not Published
• ICS: 35.020 - Information technology (IT) in general
• Technical Committee: SPN - Services and Protocols for Networks
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 22-Oct-2026
• Due Date: 27-Dec-2026
• Ref Project: ETSI ES 202 975 V3.1.1 (2026-05) - Human Factors (HF); Requirements for relay services

SIST ES 202 975 V3.1.1:2026, published by SIST, is a Human Factors standard for relay services. It specifies requirements for relay services and for invoking them over ICT networks so that users with hearing, vision, speech or cognitive limitations can communicate in real time with other users of electronic communications services. The document is written for relay service providers, communications service providers, and procurement teams that need accessible communication services.

What does SIST ES 202 975 V3.1.1:2026 specify?

SIST ES 202 975 V3.1.1:2026 specifies relay-service requirements across the service itself and the functions used to invoke it on communication platforms. It covers services delivered around the clock as well as limited-hour services, and it includes emergency communication, person-to-person communication, and multiparty conferencing.

The document is organized into:

• Clause 1-2: scope and references
• Clause 3: terms, symbols, and abbreviations
• Clause 4-8: functional requirements, service provision, interoperability, communications assistants, and user aspects
• Annexes A-F: guidance, use cases, billing and complaints, development directions, media connections, and change history

Annex	What it covers
A	Needs and rights of Communications Assistants/Interpreters
B	Use cases and their decomposition into operational steps
C	Additional considerations for relay service organization
D	Development directions
E	Relay service media connections
F	Change history

What are the key requirements of SIST ES 202 975 V3.1.1:2026?

SIST ES 202 975 V3.1.1:2026 sets requirements that matter in practice for service design, invocation, staffing, and interoperability. The main point is that a relay service must not just exist - users also need a workable way to reach it, control it, and keep the conversation going without losing access.

Relay service types and support model

Clause 4.1 recognizes several relay service types: text relay, video relay, captioned telephony, speech-to-speech relay, memory and cognitive support relay, and lipspeaking relay. Clause 4.1.1 also allows delivery by human CAs only, by human CAs supported by automated tools, or by automated CAs only for some services. In practice, procurement and service design have to match the service type to the media and communication support the users actually need.

Invocation and equal access

Clause 4.2 makes Relay Service Invocation Functions (RSIF) part of the architecture. RSIF handles detection of when relay is needed, connection and disconnection of the relay service, completion of the destination connection, and control of call-progress information. This matters because the primary user must be able to invoke relay service during setup or during an ongoing communication without losing the ability to reach the other party.

Emergency communication priority

Clause 4.3 gives emergency communication special priority. The communication to the Public Safety Answering Point (PSAP) is to start as soon as possible, even before the CA connects, and other relayed sessions may be put on hold if needed. If emergency communication starts without relay invocation but needs relay support, the PSAP must be able to bring that support into the call. In practice, emergency handling cannot depend on ordinary queue order.

Provision, quality, and reliability

Clause 5 covers organization plan, quality assurance, opening hours, answering times, queue situations, language, communication restrictions, traffic recording, service performance management, error messages, and disaster recovery. This means the provider needs procedures for live operations as well as for continuity when service delivery is disrupted.

Interoperability and addressing

Clause 6 requires relay service interaction with communications services to support interoperability, transmission quality, and addressing equivalence. It covers number-based and non-number-based addressing, caller identifier and destination address handling, and use of user equipment for communications involving relay services. For readers, this means the service must work across different address forms and preserve the ability to identify, present, or suppress identity information consistently.

Relay service invocation procedures

Clause 6.4 allows automatic invocation, manual selection, no relay service, and relay service changes during the call. The primary user can also disconnect relay service during an ongoing communication while keeping the connection with the secondary user. This is important for situations where the user starts a call one way and later decides that a different relay mode, or no relay at all, is better.

Communications assistants and user aspects

Clause 7 sets competence and conduct expectations for Communications Assistants. For text relay, the CA typing speed requirement is given; for video relay, sign language interpreting education or certification is required; automated transcoders must be sufficiently rapid and correct for the supported languages. Clause 8 adds user-profile preferences, accessibility of the user interface, and a test environment for validation and compatibility testing. In practice, users need configurable preferences, and vendors need a way to test access before deployment.

What terms does SIST ES 202 975 V3.1.1:2026 define?

• relay service - An electronic communications service that enables real-time bidirectional communication between users who use different communication modalities or need other communication support.
• primary user (of relay service) - A person using relay services because of a disability; the document treats this user as the one the relay service is primarily intended to support.
• secondary user (of relay service) - A communication user other than the primary user, participating in the communication through the relay service.
• Communications Assistant (CA) - A human operator or automatic agent that converts between modalities or provides interpretation and other communication support.
• Relay Service Invocation Functions (RSIF) - The functions that decide when relay is needed and establish, manage, or disconnect the relay connection.
• Real-Time Text (RTT) - Text conversation where characters are displayed continuously as they are entered.
• Video Relay Service (VRS) - A service that lets sign language users and voice users communicate through real-time interpretation between sign language and spoken language.

Who uses SIST ES 202 975 V3.1.1:2026?

SIST ES 202 975 V3.1.1:2026 is used by:

• relay service providers that run text, video, captioned telephony, or speech-related relay services
• communications service providers that need to support relay invocation and media interworking
• public procurement agencies that specify accessible communication services
• CA teams and sign language interpreters who deliver the service
• vendors and testers who need a compatibility and validation environment
• organizations handling emergency communications and PSAP integration

In practice, these groups use the document to define service capability, staffing, user access, and interoperability expectations.

What changed in SIST ES 202 975 V3.1.1:2026 from the previous edition?

SIST ES 202 975 V3.1.1:2026 updates ETSI ES 202 975 (V2.1.1). The change history states that the 2026 publication includes updates related to EU M/587 and Directive (EU) 2019/882.

Which standards are used with SIST ES 202 975 V3.1.1:2026?

• EN 301 549 - supplies the accessibility requirements for ICT products and services, which the user interface for relay users and CAs must meet.
• ES 204 009 - provides requirements for interoperable total conversation services and media quality expectations referenced for interoperability and transmission quality.
• EG 201 013 - provides the shared terms, abbreviations, and symbols used in the document.
• TR 102 974 - gives background research on telecommunications relay services.
• TS 101 470 - covers total conversation access to emergency services.
• TS 103 919 - covers accessibility and interoperability of emergency communications and PSAP answering.
• RFC 3261, RFC 4353, RFC 4575, RFC 4579, RFC 8825 - provide SIP and browser-based real-time communication foundations used in the implementation context.

What does the SIST ES 202 975 V3.1.1:2026 document contain?

The document contains clause-based functional requirements plus annexes with practical support material. Clause 4 includes conceptual figures for relay architectures and call flows, while Annex E provides tables of default media connections for text relay, captioned telephony, video relay, speech-to-speech, cognitive and memory support, lipspeaking, emergency communication, and video conference use. Annex B breaks use cases into operational steps and gives worked examples, including video relay, captioned telephony, speech-to-speech, cognitive support, text relay, and emergency cases. Annex A adds guidance on training, counselling, working conditions, and handling profanity or illegality, and Annex C covers billing and complaint handling.

Final draft ETSI ES 202 975 V3.1.0 (2026-03)

ETSI STANDARD
Human Factors (HF);
Requirements for relay services

2 Final draft ETSI ES 202 975 V3.1.0 (2026-03)

Reference
RES/HF-00301560
Keywords
accessibility, HF, ICT, procurement, relay, service
ETSI
650 Route des Lucioles
F-06921 Sophia Antipolis Cedex - FRANCE

Tel.: [phone]  Fax: [phone]

Siret N° 348 623 562 00017 - APE 7112B
Association à but non lucratif enregistrée à la
Sous-Préfecture de Grasse (06) N° w061004871

Important notice
The present document can be downloaded from the
ETSI Search & Browse Standards application.
The present document may be made available in electronic versions and/or in print. The content of any electronic and/or
print versions of the present document shall not be modified without the prior written authorization of ETSI. In case of any
existing or perceived difference in contents between such versions and/or in print, the prevailing version of an ETSI
deliverable is the one made publicly available in PDF format on ETSI deliver repository.
Users should be aware that the present document may be revised or have its status changed,
this information is available in the Milestones listing.
If you find errors in the present document, please send your comments to
the relevant service listed under Committee Support Staff.
If you find a security vulnerability in the present document, please report it through our
Coordinated Vulnerability Disclosure (CVD) program.
Notice of disclaimer & limitation of liability
The information provided in the present deliverable is directed solely to professionals who have the appropriate degree of
experience to understand and interpret its content in accordance with generally accepted engineering or
other professional standard and applicable regulations.
No recommendation as to products and services or vendors is made or should be implied.
No representation or warranty is made that this deliverable is technically accurate or sufficient or conforms to any law
and/or governmental rule and/or regulation and further, no representation or warranty is made of merchantability or fitness
for any particular purpose or against infringement of intellectual property rights.
In no event shall ETSI be held liable for loss of profits or any other incidental or consequential damages.

Any software contained in this deliverable is provided "AS IS" with no warranties, express or implied, including but not
limited to, the warranties of merchantability, fitness for a particular purpose and non-infringement of intellectual property
rights and ETSI shall not be held liable in any event for any damages whatsoever (including, without limitation, damages
for loss of profits, business interruption, loss of information, or any other pecuniary loss) arising out of or related to the use
of or inability to use the software.
Copyright Notification
No part may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and
microfilm except as authorized by written permission of ETSI.
The content of the PDF version shall not be modified without the written authorization of ETSI.
The copyright and the foregoing restriction extend to reproduction in all media.

© ETSI 2026.
All rights reserved.
ETSI
3 Final draft ETSI ES 202 975 V3.1.0 (2026-03)
Contents
Contents . 3
Intellectual Property Rights . 6
Foreword. 6
Modal verbs terminology . 6
Introduction . 6
1 Scope . 7
2 References . 7
2.1 Normative references . 7
2.2 Informative references . 7
3 Definitions of terms, symbols and abbreviations . 8
3.1 Terms . 8
3.2 Symbols . 10
3.3 Abbreviations . 10
4 General functional requirements on relay services and use cases . 11
4.1 Relay service types . 11
4.1.1 General . 11
4.1.2 Text relay service . 12
4.1.3 Video relay service . 12
4.1.4 Captioned telephony services . 13
4.1.5 Speech to speech relay service . 13
4.1.6 Memory and cognitive support relay service . 13
4.1.7 Lipspeaking relay service . 13
4.2 Functional requirements for relay service invocation . 13
4.2.1 Functional architecture and invocation of relay services . 13
4.2.2 Relay service invocation for equal access to communication . 15
4.3 Relay service use cases . 18
4.3.1 Generic outline of relay service use cases . 18
4.3.2 Communication with other individual users (secondary users) . 18
4.3.3 Communication in a multiparty conference . 18
4.3.4 Relay service use during emergency communication . 18
4.3.5 Queueing use case . 19
4.3.6 Interacting relay services . 19
4.3.7 Relaying automated alternatives to voice-based services . 19
4.3.8 Remote interpreting . 19
4.3.9 Call progress information and other audible signals . 19
5 Requirements for provision of relay service . 20
5.1 Relay service organization and quality . 20
5.1.1 Organization plan . 20
5.1.2 Quality assurance . 20
5.2 Relay service delivery . 20
5.2.1 Opening hours . 20
5.2.2 Answering times . 21
5.2.3 Queue situations . 21
5.2.4 Language . 21
5.2.5 Communication restrictions . 21
5.2.6 Hold . 21
5.2.7 Traffic recording . 22
5.2.8 Communication with automated alternatives to voice-based services . 22
5.3 Relay service reliability . 22
5.3.1 Availability . 22
5.3.2 Service performance management . 23
5.3.3 Error messages . 23
ETSI
4 Final draft ETSI ES 202 975 V3.1.0 (2026-03)
5.3.4 Disaster recovery plan . 23
6 Interaction of relay service , relay service invocation functions and communications services . 24
6.1 Ensuring the interoperability. 24
6.1.1 General . 24
6.1.2 Communication establishment . 24
6.1.3 Media specifications . 24
6.1.4 Web access . 25
6.1.5 Access for ICT unsupported by the relay service . 25
6.2 Ensuring the transmission quality . 25
6.3 Ensuring addressing equivalence . 26
6.3.1 Supported forms of addressing . 26
6.3.2 Equivalent access and use of communications service for the primary users . 26
6.3.3 Use of user equipment for communications involving relay services . 27
6.4 Relay service invocation for communication with other individual users . 27
6.4.1 Purpose of relay service invocation . 27
6.4.2 Relay service invocation functions . 27
6.4.3 Relay service invocation procedure . 28
6.5 Invocation related to emergency communication . 30
6.5.1 Relay service invocation for emergency communication . 30
6.5.2 Relay service support requested by the emergency communications PSAP . 31
6.6 Relay service invocation in a multiparty conference . 31
6.6.1 General about use of relay service in a multiparty conference . 31
6.6.2 Relayed communication within the same conference connection . 31
6.6.3 Relayed communication via a separate connection . 32
6.7 Provision of supplementary services . 32
6.7.1 General . 32
6.7.2 Conference and 3-party communication . 32
6.7.3 Relay service invocation performance management . 32
7 Requirements on communications assistants . 33
7.1 Communications assistants and sign language interpreters . 33
7.2 Proficiency requirements (including requirements on automated CAs) . 33
7.3 Procedures. 33
7.3.1 Information . 33
7.3.2 Freedom from bias . 33
7.3.3 Assistance . 33
7.3.4 Sign language interpreters' code of practice . 34
7.3.5 Neutrality . 34
7.3.6 Accuracy . 34
7.4 Confidentiality . 34
7.4.1 Content disclosure . 34
7.4.2 Secrecy . 34
7.4.3 Privacy . 34
7.4.4 Emergencies . 34
8 User aspects . 34
8.1 General . 34
8.2 Primary user preferences . 35
8.3 User interface considerations . 35
8.4 Information about relay service . 35
8.4.1 General information . 35
8.4.2 Accessibility of the information provided during the use of the service (including interoperability
with end user products and service) . 35
8.4.3 Technical information . 35
8.5 Test environment . 36
Annex A (informative): Needs and Rights of Communication Assistants/Interpreters . 37
A.1 Training requirements . 37
A.2 Counselling . 37
A.3 Working conditions . 37
ETSI
5 Final draft ETSI ES 202 975 V3.1.0 (2026-03)
A.4 Profanity, obscenity and illegality . 37
A.4.1 Profanity and obscenity in conversations . 37
A.4.2 Obscenity directed to the CA . 37
A.4.3 Illegality . 38
Annex B (informative): Use cases and their decomposition in operational steps . 39
B.1 Operational steps . 39
B.2 Use case examples . 41
B.2.1 General . 41
B.2.2 Video relay user in different communication situations . 41
B.2.2.1 Video relay user requesting relay for a communication to mobile voice user . 41
B.2.2.2 Video relay user unable to answer communication by video . 42
B.2.3 Video relay user receiving an incoming communication starting with RTT . 42
B.2.4 Captioned telephony user answering . 43
B.2.4.1 Captioned telephony user answering known person without relay service . 43
B.2.4.2 Captioned telephony user answering and including captioned telephony relay service . 43
B.2.5 Speech‑to‑Speech relay user initiating a communication to a bank employee . 44
B.2.6 Cognitive and memory support relay user initiating a call to a pharmacy . 44
B.2.7 Text relay user initiating a call to a delivery driver . 45
B.2.8 Emergency communication getting text relay service invoked by PSAP . 45
Annex C (informative): Additional considerations for relay services' organization . 47
C.1 Billing . 47
C.2 Complaints and inquiry handling . 47
Annex D (informative): Development directions . 48
D.1 Development directions in general . 48
D.2 Move to IP based communications . 48
D.3 Increasing use of non-number based electronic communication . 48
D.4 Increasing use of digital conferencing. 48
D.5 Reducing use of text and video relay services and increasing use of captioned telephony. 49
D.6 Maintained need for app and web based video relay access . 49
D.7 Smooth invocation via user's own number or address . 49
D.8 Desired development in speed of answer for non-speaking users . 49
Annex E (informative): Relay service media connections . 51
E.1 Tables for relay service media connections . 51
Annex F (informative): Change history . 55
History . 56

ETSI
6 Final draft ETSI ES 202 975 V3.1.0 (2026-03)
Intellectual Property Rights
Essential patents
IPRs essential or potentially essential to normative deliverables may have been declared to ETSI. The declarations
pertaining to these essential IPRs, if any, are publicly available for ETSI members and non-members, and can be
found in ETSI SR 000 314: "Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to
ETSI in respect of ETSI standards", which is available from the ETSI Secretariat. Latest updates are available on the
ETSI IPR online database.
Pursuant to the ETSI Directives including the ETSI IPR Policy, no investigation regarding the essentiality of IPRs,
including IPR searches, has been carried out by ETSI. No guarantee can be given as to the existence of other IPRs not
referenced in ETSI SR 000 314 (or the updates on the ETSI Web server) which are, or may be, or may become,
essential to the present document.
Trademarks
The present document may include trademarks and/or tradenames which are asserted and/or registered by their owners.
ETSI claims no ownership of these except for any which are indicated as being the property of ETSI, and conveys no
right to use or reproduce any trademark and/or tradename. Mention of those trademarks in the present document does
not constitute an endorsement by ETSI of products, services or organizations associated with those trademarks.
DECT™, PLUGTESTS™, UMTS™ and the ETSI logo are trademarks of ETSI registered for the benefit of its
Members. 3GPP™, LTE™ and 5G™ logo are trademarks of ETSI registered for the benefit of its Members and of the
3GPP Organizational Partners. oneM2M™ logo is a trademark of ETSI registered for the benefit of its Members and of ®
the oneM2M Partners. GSM and the GSM logo are trademarks registered and owned by the GSM Association.
Foreword
This final draft ETSI Standard (ES) has been produced by ETSI Technical Committee Human Factors (HF), and is now
submitted for the Vote phase of the ETSI Membership Approval Procedure (MAP).
Modal verbs terminology
In the present document "shall", "shall not", "should", "should not", "may", "need not", "will", "will not", "can" and
"cannot" are to be interpreted as described in clause 3.2 of the ETSI Drafting Rules (Verbal forms for the expression of
provisions).
"must" and "must not" are NOT allowed in ETSI deliverables except when used in direct citation.
Introduction
The present document is an update of ETSI ES 202 975 (V2.1.1) [i.2].
Further significant background and research information about relay services and the development of the original
version of the present document can be found in ETSI TR 102 974 [i.3].
Recent development in the area has been taken into careful consideration.
The present document is intended to support the procurement and delivery of accessible and usable relay services
including their provision and invocation. It may be useful for both public procurement agencies, relay service and
communications service providers.
ETSI
7 Final draft ETSI ES 202 975 V3.1.0 (2026-03)
1 Scope
The present document specifies requirements for relay services, covering both the relay service and its invocation over
ICT networks.
The present document applies to all types of relay services that enable users with functional limitations related to
hearing, vision, speech or cognition, to communicate in real time with other users of electronic communications
services. The present document applies to, inter alia, text relay services, speech-to-speech relay services, video relay
services, captioned telephony services, lipspeaking relay services and cognitive and memory support services.
Functional requirements for the invocation of relay services within communication platforms are included.
Implementation of the invocation functions is needed to ensure equal access to electronic communications services.
Included use cases cover emergency communications, communication with other individual users, and the integration of
relay services in multiparty conferencing scenarios. Requirements are specified for services provided on a 24/7 basis, as
well as for limited-hour services.
2 References
2.1 Normative references
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
referenced document (including any amendments) applies.
Referenced documents which are not found to be publicly available in the expected location might be found in the
ETSI docbox.
NOTE: While any hyperlinks included in this clause were valid at the time of publication, ETSI cannot guarantee
their long-term validity.
The following referenced documents are necessary for the application of the present document.
[1] ETSI/CEN/CENELEC EN 301 549 (V3.2.1): "Accessibility requirements for ICT products and
services".
[2] ETSI ES 204 009: "Human Factors (HF); Requirements for interoperable total conversation
services".
2.2 Informative references
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
referenced document (including any amendments) applies.
NOTE: While any hyperlinks included in this clause were valid at the time of publication, ETSI cannot guarantee
their long-term validity.
The following referenced documents may be useful in implementing an ETSI deliverable or add to the reader's
understanding, but are not required for conformance to the present document.
[i.1] Directive (EU) 2019/882 of the European Parliament and of the Council of 17 April 2019 on the
accessibility requirements for products and services.
[i.2] ETSI ES 202 975 (V2.1.1): "Human Factors (HF); Requirements for relay services".
[i.3] ETSI TR 102 974: "Human Factors (HF); Telecommunications relay services".
[i.4] ETSI EG 201 013: "Human Factors (HF); Definitions, abbreviations and symbols".
[i.5] ETSI TR 102 202 (V1.1.2): "Human Factors (HF); Human Factors of work in call centres".
ETSI
8 Final draft ETSI ES 202 975 V3.1.0 (2026-03)
[i.6] ETSI TS 101 470 (V1.1.1): "Emergency Communications (EMTEL); Total Conversation Access
to Emergency Services".
[i.7] ETSI TS 103 919: "Emergency Communications (EMTEL); Accessibility and interoperability of
emergency communications and for the answering of emergency communications by the public
safety answering points (PSAPs) (including to the single European Emergency number 112)".
[i.8] IETF RFC 3261: "Session Initiation Protocol".
[i.9] IETF RFC 4353: "A Framework for Conferencing with the Session Initiation Protocol (SIP)".
[i.10] IETF RFC 4575: "A Session Initiation Protocol (SIP) Event Package for Conference State".
[i.11] IETF RFC 4579: "Session Initiation Protocol (SIP) Call Control - Conferencing for User Agents".
[i.12] IETF RFC 8825: "Overview: Real-Time Protocols for Browser-Based Applications".
[i.13] FCC-25-79A1: "Notice on proposed rule making in the Matter of Telecommunications Relay
Services and Speech-to-Speech Services for Individuals with Hearing and Speech Disabilities
Speech-to-Speech and Internet Protocol (IP) Speech-to-Speech Telecommunications Relay
Services", Adopted: November 20, 2025, Released: November 21, 2025.
3 Definitions of terms, symbols and abbreviations
3.1 Terms
For the purposes of the present document, the terms given in ETSI EG 201 013 [i.4] and the following apply:
caller identifier (caller ID): information presented to the recipient of an incoming request for a communication session
(such as a phone call, video call, or messaging) that uniquely specifies the identity of the originating party
captioned telephony: service which converts speech to real-time text towards the primary user while letting voice
through in both directions
NOTE 1: When provided for VoIP, it is called IP Captioned Telephony Service or IPCTS in the US.
NOTE 2: A conversation may have more than one primary user.
commissioning agent: person or body that procures or otherwise puts an obligation on a service provider to deliver a
relay service, with respect to provision of relay services and/or their invocation
Communications Assistant (CA): human operator or automatic agent providing conversion between different
modalities and/or interpretation or other communication support between the primary and secondary user
NOTE: Other terms to refer to human CAs that can be encountered include among others: "interpreter",
"operator", "call handler", etc.
destination address: addressable identity specified by the initiating participant to initiate a session
ICT network: technology and resources supporting the connection and operation of interconnected ICT
Information and Communication Technology (ICT): technology, equipment, or interconnected system or subsystem
of equipment for which the principal function is the creation, conversion, duplication, automatic acquisition, storage,
analysis, evaluation, manipulation, management, movement, control, display, switching, interchange, transmission,
reception, or broadcast of data or information
NOTE: Examples of ICT are electronic content, telecommunications products, computers and ancillary
equipment, software, information kiosks and transaction machines, videos, IT services, and multifunction
office machines which copy, scan, and fax documents.
interacting relay services: relay services connected through a common voice path, in order to provide connectivity and
modality conversion between two (or several) primary relay service users
ETSI
9 Final draft ETSI ES 202 975 V3.1.0 (2026-03)
lost communication: communication that cannot be serviced as expected by the users, for reasons internal to the
service and outside of control of the users
modality: way in which communication is perceived or is expressed
NOTE: The most valid examples for the present document are signed (= using sign language), written and spoken
modalities.
primary user (of relay service): person who is using relay services because of a disability
NOTE 1: Primary users are the target users of relay services who need some communication modality conversion
or other communication support in order to communicate with voice users.
NOTE 2: A primary user can initiate and receive communications.
Real-Time Text (RTT): form of text conversation in point-to-point situations or in multipoint conferencing where the
text being entered is displayed in such a way that the communication is perceived by the user as being continuous on a
character-by-character basis
relay service: electronic communications service which enables real-time bidirectional communication between end-
users who use different communication modalities or need other communication support
NOTE: This is typically achieved by providing conversion between different communication modalities such as
text, sign, and speech, or by providing the needed cognitive support for end-users during the
communication, by a human operator or automatic means.
Relay Service Invocation Functions (RSIF): set of functions related to decisions, communication establishments and
media interconnections between users and a relay service
NOTE: The implementation of RSIF may be distributed between communications services, primary user
equipment and relay service.
relay service user: primary or secondary user of a relay service
secondary user (of relay service): communication user other than a primary user, participating in a communication
facilitated by a relay service that is invoked based on the primary user's preferences or actions
NOTE 1: A secondary user may initiate or answer a communication in which a relay service is invoked by primary
user's preferences or actions.
NOTE 2: A secondary user may also be another primary user participating in a communication via another relayed
communication.
sign language interpreter: person working in a video relay service with sign language interpretation
NOTE: Also called "interpreter".
speech to speech relay service: electronic communications service that enables electronic communication users with
speech disabilities and other users to interact by providing skilled communications assistance between them
text relay service: electronic communications service that enables users of user equipment with RTT capabilities and
users of user equipment with voice capabilities to interact by providing conversion between the two communication
modalities in substantially real time
NOTE: This conversion is provided by a CA.
total conversation: audio-visual conversation service providing bidirectional symmetric real-time transfer of motion
video, text and voice between users in two or more locations
Video Relay Service (VRS): service that enables communication between sign language users and voice
communication users by providing real-time interpretation between sign language and spoken language
NOTE: This conversion (interpretation) is normally provided by a sign language interpreter.
ETSI
10 Final draft ETSI ES 202 975 V3.1.0 (2026-03)
3.2 Symbols
For the purposes of the present document, the following visual symbols apply to indicate the three communication
media:
Table 3.1: Visual symbols used to represent communication media
Symbol Description
Audio communication
Real-time text communication
Video communication
For the purposes of the present document, the following visual symbols apply to indicate the three communication
modalities:
Table 3.2: Visual symbols used to represent communication modalities
Symbol Description
Spoken
Written
Signed
3.3 Abbreviations
For the purposes of the present document, the following abbreviations apply:
CA Communications Assistant
FCC Federal Communications Commission
GSM Global System for Mobile communication
HTML HyperText Markup Language
ICT Information and Communication Technology
IMS IP Multimedia Sub-system
IP Internet Protocol
IVR Interactive Voice Response
NPRM Notice of Proposed RuleMaking
PSAP Public Safety Answering Point
RSIF Relay Service Invocation Functions
RTT Real-Time Text
ETSI
11 Final draft ETSI ES 202 975 V3.1.0 (2026-03)
SIP Session Initiation Protocol
StT Speech-to-text
TtS Text-to-speech
VoIP Voice over IP
VRS Video Relay Service
WebRTC Real Time Communication in Web Browser
4 General functional requirements on relay services
and use cases
4.1 Relay service types
4.1.1 General
A relay service is an ICT service, as outlined in Figure 4.1, that enables users of different communication modalities to
interact by providing conversion between the modalities.
The invocation of relay service in ICT communication is handled by Relay Service Invocation Functions (RSIF) which
may be provided separately but interoperating with the relay service.
The connections in Figure 4.1 represent general session and media connections and do not show the technological
routes of the connections or points of media mixing.
The user who makes use of a relay service because of a disability is called primary user in the present document, and
another user involved in the communication is called secondary user.
NOTE: Both primary and secondary users can initiate outgoing communications as well as receive incoming
communications where relay service is invoked.

Figure 4.1: Electronic communication supported by a relay service
EXAMPLE: The relay service is facilitated by a Communications Assistant (CA), converting (interpreting)
real-time communication between a user who has no or limited use of voice communication
(e.g. sign language user or text user) and a voice communications user. The service can be
provided by a relay service provider over any form of connection, for example over a mobile
network or via an IP connection where the text/video/audio device can be based on mainstream
ICT equipment. The relay service invocation functions connect the users and the relay service.
ETSI
12 Final draft ETSI ES 202 975 V3.1.0 (2026-03)
The aim of a relay service is to support users with disabilities in their communication with other users. The relay service
support may be provided within the same service or across interoperating services and be delivered by:
• human Communication Assistant (CA) only
• human CA supported by automated CA (for example, speech-to-text tools)
• automated CA only - at the time of writing of the present document, relay services providing text relay and
captioned telephony are available with automated CA only
The continuation of clause 4.1 specifies the recognized types of relay services. Each type makes use of a typical default
set of media connections and provides the relevant conversions or other communication support. A set of tables in
clause E.1 of the present document shows these media connections and the needed conversions or other communication
support.
4.1.2 Text relay service
A text relay service shall enable the conversion:
• from real-time text, coming from user equipment with RTT capabilities, to speech delivered to user equipment
with audio communication capabilities
• from speech, coming from user equipment with audio communication capabilities, to real-time text delivered
to user equipment with RTT capabilities
NOTE 1: The conversion between the two communication modalities may be provided by a human CA, by
automated speech-to-text/text-to-speech technologies, or semi-automated conversion where a
communication assistant is supported by automated speech-to-text/text-to-speech technologies.
The following variations from this basic operation shall be provided, either by pre-defined user preference configuration
or by requesting a variation at the beginning of the communication:
• No conversion towards the text primary user who can hear, i.e. the primary user sends text, but can hear
directly the secondary user.
NOTE 2: This variation of the text relay service is sometimes referred to as Hearing Carry Over (HCO).
• No conversion from the text primary user who can talk, i.e. the primary user talks, but the speech of the
secondary user is being converted to text for the primary user to read.
NOTE 3: This variation of the text relay service is sometimes referred to as Voice Carry Over (VCO).
4.1.3 Video relay service
A video relay service shall enable communication between sign language users and voice communication users by
providing real-time interpretation between sign language and spoken language.
NOTE 1: This conv
...

ETSI STANDARD
Human Factors (HF);
Requirements for relay services

2 ETSI ES 202 975 V3.1.1 (2026-05)

Reference
RES/HF-00301560
Keywords
accessibility, HF, ICT, procurement, relay, service
ETSI
650 Route des Lucioles
F-06921 Sophia Antipolis Cedex - FRANCE

Tel.: [phone]  Fax: [phone]

Siret N° 348 623 562 00017 - APE 7112B
Association à but non lucratif enregistrée à la
Sous-Préfecture de Grasse (06) N° w061004871

Important notice
The present document can be downloaded from the
ETSI Search & Browse Standards application.
The present document may be made available in electronic versions and/or in print. The content of any electronic and/or
print versions of the present document shall not be modified without the prior written authorization of ETSI. In case of any
existing or perceived difference in contents between such versions and/or in print, the prevailing version of an ETSI
deliverable is the one made publicly available in PDF format on ETSI deliver repository.
Users should be aware that the present document may be revised or have its status changed,
this information is available in the Milestones listing.
If you find errors in the present document, please send your comments to
the relevant service listed under Committee Support Staff.
If you find a security vulnerability in the present document, please report it through our
Coordinated Vulnerability Disclosure (CVD) program.
Notice of disclaimer & limitation of liability
The information provided in the present deliverable is directed solely to professionals who have the appropriate degree of
experience to understand and interpret its content in accordance with generally accepted engineering or
other professional standard and applicable regulations.
No recommendation as to products and services or vendors is made or should be implied.
No representation or warranty is made that this deliverable is technically accurate or sufficient or conforms to any law
and/or governmental rule and/or regulation and further, no representation or warranty is made of merchantability or fitness
for any particular purpose or against infringement of intellectual property rights.
In no event shall ETSI be held liable for loss of profits or any other incidental or consequential damages.

Any software contained in this deliverable is provided "AS IS" with no warranties, express or implied, including but not
limited to, the warranties of merchantability, fitness for a particular purpose and non-infringement of intellectual property
rights and ETSI shall not be held liable in any event for any damages whatsoever (including, without limitation, damages
for loss of profits, business interruption, loss of information, or any other pecuniary loss) arising out of or related to the use
of or inability to use the software.
Copyright Notification
No part may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and
microfilm except as authorized by written permission of ETSI.
The content of the PDF version shall not be modified without the written authorization of ETSI.
The copyright and the foregoing restriction extend to reproduction in all media.

© ETSI 2026.
All rights reserved.
ETSI
3 ETSI ES 202 975 V3.1.1 (2026-05)
Contents
Contents . 3
Intellectual Property Rights . 6
Foreword. 6
Modal verbs terminology . 6
Introduction . 6
1 Scope . 7
2 References . 7
2.1 Normative references . 7
2.2 Informative references . 7
3 Definitions of terms, symbols and abbreviations . 8
3.1 Terms . 8
3.2 Symbols . 10
3.3 Abbreviations . 10
4 General functional requirements on relay services and use cases . 11
4.1 Relay service types . 11
4.1.1 General . 11
4.1.2 Text relay service . 12
4.1.3 Video relay service . 12
4.1.4 Captioned telephony services . 13
4.1.5 Speech to speech relay service . 13
4.1.6 Memory and cognitive support relay service . 13
4.1.7 Lipspeaking relay service . 13
4.2 Functional requirements for relay service invocation . 13
4.2.1 Functional architecture and invocation of relay services . 13
4.2.2 Relay service invocation for equal access to communication . 15
4.3 Relay service use cases . 18
4.3.1 Generic outline of relay service use cases . 18
4.3.2 Communication with other individual users (secondary users) . 18
4.3.3 Communication in a multiparty conference . 18
4.3.4 Relay service use during emergency communication . 18
4.3.5 Queueing use case . 19
4.3.6 Interacting relay services . 19
4.3.7 Relaying automated alternatives to voice-based services . 19
4.3.8 Remote interpreting . 19
4.3.9 Call progress information and other audible signals . 19
5 Requirements for provision of relay service . 20
5.1 Relay service organization and quality . 20
5.1.1 Organization plan . 20
5.1.2 Quality assurance . 20
5.2 Relay service delivery . 20
5.2.1 Opening hours . 20
5.2.2 Answering times . 21
5.2.3 Queue situations . 21
5.2.4 Language . 21
5.2.5 Communication restrictions . 21
5.2.6 Hold . 21
5.2.7 Traffic recording . 22
5.2.8 Communication with automated alternatives to voice-based services . 22
5.3 Relay service reliability . 22
5.3.1 Availability . 22
5.3.2 Service performance management . 23
5.3.3 Error messages . 23
ETSI
4 ETSI ES 202 975 V3.1.1 (2026-05)
5.3.4 Disaster recovery plan . 23
6 Interaction of relay service , relay service invocation functions and communications services . 24
6.1 Ensuring the interoperability. 24
6.1.1 General . 24
6.1.2 Communication establishment . 24
6.1.3 Media specifications . 24
6.1.4 Web access . 25
6.1.5 Access for ICT unsupported by the relay service . 25
6.2 Ensuring the transmission quality . 25
6.3 Ensuring addressing equivalence . 26
6.3.1 Supported forms of addressing . 26
6.3.2 Equivalent access and use of communications service for the primary users . 26
6.3.3 Use of user equipment for communications involving relay services . 27
6.4 Relay service invocation for communication with other individual users . 27
6.4.1 Purpose of relay service invocation . 27
6.4.2 Relay service invocation functions . 27
6.4.3 Relay service invocation procedure . 28
6.5 Invocation related to emergency communication . 30
6.5.1 Relay service invocation for emergency communication . 30
6.5.2 Relay service support requested by the emergency communications PSAP . 31
6.6 Relay service invocation in a multiparty conference . 31
6.6.1 General about use of relay service in a multiparty conference . 31
6.6.2 Relayed communication within the same conference connection . 31
6.6.3 Relayed communication via a separate connection . 32
6.7 Provision of supplementary services . 32
6.7.1 General . 32
6.7.2 Conference and 3-party communication . 32
6.7.3 Relay service invocation performance management . 32
7 Requirements on communications assistants . 33
7.1 Communications assistants and sign language interpreters . 33
7.2 Proficiency requirements (including requirements on automated CAs) . 33
7.3 Procedures. 33
7.3.1 Information . 33
7.3.2 Freedom from bias . 33
7.3.3 Assistance . 33
7.3.4 Sign language interpreters' code of practice . 34
7.3.5 Neutrality . 34
7.3.6 Accuracy . 34
7.4 Confidentiality . 34
7.4.1 Content disclosure . 34
7.4.2 Secrecy . 34
7.4.3 Privacy . 34
7.4.4 Emergencies . 34
8 User aspects . 34
8.1 General . 34
8.2 Primary user preferences . 35
8.3 User interface considerations . 35
8.4 Information about relay service . 35
8.4.1 General information . 35
8.4.2 Accessibility of the information provided during the use of the service (including interoperability
with end user products and service) . 35
8.4.3 Technical information . 35
8.5 Test environment . 36
Annex A (informative): Needs and Rights of Communication Assistants/Interpreters . 37
A.1 Training requirements . 37
A.2 Counselling . 37
A.3 Working conditions . 37
ETSI
5 ETSI ES 202 975 V3.1.1 (2026-05)
A.4 Profanity, obscenity and illegality . 37
A.4.1 Profanity and obscenity in conversations . 37
A.4.2 Obscenity directed to the CA . 37
A.4.3 Illegality . 38
Annex B (informative): Use cases and their decomposition in operational steps . 39
B.1 Operational steps . 39
B.2 Use case examples . 41
B.2.1 General . 41
B.2.2 Video relay user in different communication situations . 41
B.2.2.1 Video relay user requesting relay for a communication to mobile voice user . 41
B.2.2.2 Video relay user unable to answer communication by video . 42
B.2.3 Video relay user receiving an incoming communication starting with RTT . 42
B.2.4 Captioned telephony user answering . 43
B.2.4.1 Captioned telephony user answering known person without relay service . 43
B.2.4.2 Captioned telephony user answering and including captioned telephony relay service . 43
B.2.5 Speech‑to‑Speech relay user initiating a communication to a bank employee . 44
B.2.6 Cognitive and memory support relay user initiating a call to a pharmacy . 44
B.2.7 Text relay user initiating a call to a delivery driver . 45
B.2.8 Emergency communication getting text relay service invoked by PSAP . 45
Annex C (informative): Additional considerations for relay services' organization . 47
C.1 Billing . 47
C.2 Complaints and inquiry handling . 47
Annex D (informative): Development directions . 48
D.1 Development directions in general . 48
D.2 Move to IP based communications . 48
D.3 Increasing use of non-number based electronic communication . 48
D.4 Increasing use of digital conferencing. 48
D.5 Reducing use of text and video relay services and increasing use of captioned telephony. 49
D.6 Maintained need for app and web based video relay access . 49
D.7 Smooth invocation via user's own number or address . 49
D.8 Desired development in speed of answer for non-speaking users . 49
Annex E (informative): Relay service media connections . 51
E.1 Tables for relay service media connections . 51
Annex F (informative): Change history . 55
History . 56

ETSI
6 ETSI ES 202 975 V3.1.1 (2026-05)
Intellectual Property Rights
Essential patents
IPRs essential or potentially essential to normative deliverables may have been declared to ETSI. The declarations
pertaining to these essential IPRs, if any, are publicly available for ETSI members and non-members, and can be
found in ETSI SR 000 314: "Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to
ETSI in respect of ETSI standards", which is available from the ETSI Secretariat. Latest updates are available on the
ETSI IPR online database.
Pursuant to the ETSI Directives including the ETSI IPR Policy, no investigation regarding the essentiality of IPRs,
including IPR searches, has been carried out by ETSI. No guarantee can be given as to the existence of other IPRs not
referenced in ETSI SR 000 314 (or the updates on the ETSI Web server) which are, or may be, or may become,
essential to the present document.
Trademarks
The present document may include trademarks and/or tradenames which are asserted and/or registered by their owners.
ETSI claims no ownership of these except for any which are indicated as being the property of ETSI, and conveys no
right to use or reproduce any trademark and/or tradename. Mention of those trademarks in the present document does
not constitute an endorsement by ETSI of products, services or organizations associated with those trademarks.
DECT™, PLUGTESTS™, UMTS™ and the ETSI logo are trademarks of ETSI registered for the benefit of its
Members. 3GPP™, LTE™ and 5G™ logo are trademarks of ETSI registered for the benefit of its Members and of the
3GPP Organizational Partners. oneM2M™ logo is a trademark of ETSI registered for the benefit of its Members and of ®
the oneM2M Partners. GSM and the GSM logo are trademarks registered and owned by the GSM Association.
Foreword
This ETSI Standard (ES) has been produced by ETSI Technical Committee Human Factors (HF).
Modal verbs terminology
In the present document "shall", "shall not", "should", "should not", "may", "need not", "will", "will not", "can" and
"cannot" are to be interpreted as described in clause 3.2 of the ETSI Drafting Rules (Verbal forms for the expression of
provisions).
"must" and "must not" are NOT allowed in ETSI deliverables except when used in direct citation.
Introduction
The present document is an update of ETSI ES 202 975 (V2.1.1) [i.2].
Further significant background and research information about relay services and the development of the original
version of the present document can be found in ETSI TR 102 974 [i.3].
Recent development in the area has been taken into careful consideration.
The present document is intended to support the procurement and delivery of accessible and usable relay services
including their provision and invocation. It may be useful for both public procurement agencies, relay service and
communications service providers.
ETSI
7 ETSI ES 202 975 V3.1.1 (2026-05)
1 Scope
The present document specifies requirements for relay services, covering both the relay service and its invocation over
ICT networks.
The present document applies to all types of relay services that enable users with functional limitations related to
hearing, vision, speech or cognition, to communicate in real time with other users of electronic communications
services. The present document applies to, inter alia, text relay services, speech-to-speech relay services, video relay
services, captioned telephony services, lipspeaking relay services and cognitive and memory support services.
Functional requirements for the invocation of relay services within communication platforms are included.
Implementation of the invocation functions is needed to ensure equal access to electronic communications services.
Included use cases cover emergency communications, communication with other individual users, and the integration of
relay services in multiparty conferencing scenarios. Requirements are specified for services provided on a 24/7 basis, as
well as for limited-hour services.
2 References
2.1 Normative references
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
referenced document (including any amendments) applies.
Referenced documents which are not found to be publicly available in the expected location might be found in the
ETSI docbox.
NOTE: While any hyperlinks included in this clause were valid at the time of publication, ETSI cannot guarantee
their long-term validity.
The following referenced documents are necessary for the application of the present document.
[1] ETSI/CEN/CENELEC EN 301 549 (V3.2.1): "Accessibility requirements for ICT products and
services".
[2] ETSI ES 204 009: "Human Factors (HF); Requirements for interoperable total conversation
services".
2.2 Informative references
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
referenced document (including any amendments) applies.
NOTE: While any hyperlinks included in this clause were valid at the time of publication, ETSI cannot guarantee
their long-term validity.
The following referenced documents may be useful in implementing an ETSI deliverable or add to the reader's
understanding, but are not required for conformance to the present document.
[i.1] Directive (EU) 2019/882 of the European Parliament and of the Council of 17 April 2019 on the
accessibility requirements for products and services.
[i.2] ETSI ES 202 975 (V2.1.1): "Human Factors (HF); Requirements for relay services".
[i.3] ETSI TR 102 974: "Human Factors (HF); Telecommunications relay services".
[i.4] ETSI EG 201 013: "Human Factors (HF); Definitions, abbreviations and symbols".
[i.5] ETSI TR 102 202 (V1.1.2): "Human Factors (HF); Human Factors of work in call centres".
ETSI
8 ETSI ES 202 975 V3.1.1 (2026-05)
[i.6] ETSI TS 101 470 (V1.1.1): "Emergency Communications (EMTEL); Total Conversation Access
to Emergency Services".
[i.7] ETSI TS 103 919: "Emergency Communications (EMTEL); Accessibility and interoperability of
emergency communications and for the answering of emergency communications by the public
safety answering points (PSAPs) (including to the single European Emergency number 112)".
[i.8] IETF RFC 3261: "Session Initiation Protocol".
[i.9] IETF RFC 4353: "A Framework for Conferencing with the Session Initiation Protocol (SIP)".
[i.10] IETF RFC 4575: "A Session Initiation Protocol (SIP) Event Package for Conference State".
[i.11] IETF RFC 4579: "Session Initiation Protocol (SIP) Call Control - Conferencing for User Agents".
[i.12] IETF RFC 8825: "Overview: Real-Time Protocols for Browser-Based Applications".
[i.13] FCC-25-79A1: "Notice on proposed rule making in the Matter of Telecommunications Relay
Services and Speech-to-Speech Services for Individuals with Hearing and Speech Disabilities
Speech-to-Speech and Internet Protocol (IP) Speech-to-Speech Telecommunications Relay
Services", Adopted: November 20, 2025, Released: November 21, 2025.
3 Definitions of terms, symbols and abbreviations
3.1 Terms
For the purposes of the present document, the terms given in ETSI EG 201 013 [i.4] and the following apply:
caller identifier (caller ID): information presented to the recipient of an incoming request for a communication session
(such as a phone call, video call, or messaging) that uniquely specifies the identity of the originating party
captioned telephony: service which converts speech to real-time text towards the primary user while letting voice
through in both directions
NOTE 1: When provided for VoIP, it is called IP Captioned Telephony Service or IPCTS in the US.
NOTE 2: A conversation may have more than one primary user.
commissioning agent: person or body that procures or otherwise puts an obligation on a service provider to deliver a
relay service, with respect to provision of relay services and/or their invocation
Communications Assistant (CA): human operator or automatic agent providing conversion between different
modalities and/or interpretation or other communication support between the primary and secondary user
NOTE: Other terms to refer to human CAs that can be encountered include among others: "interpreter",
"operator", "call handler", etc.
destination address: addressable identity specified by the initiating participant to initiate a session
ICT network: technology and resources supporting the connection and operation of interconnected ICT
Information and Communication Technology (ICT): technology, equipment, or interconnected system or subsystem
of equipment for which the principal function is the creation, conversion, duplication, automatic acquisition, storage,
analysis, evaluation, manipulation, management, movement, control, display, switching, interchange, transmission,
reception, or broadcast of data or information
NOTE: Examples of ICT are electronic content, telecommunications products, computers and ancillary
equipment, software, information kiosks and transaction machines, videos, IT services, and multifunction
office machines which copy, scan, and fax documents.
interacting relay services: relay services connected through a common voice path, in order to provide connectivity and
modality conversion between two (or several) primary relay service users
ETSI
9 ETSI ES 202 975 V3.1.1 (2026-05)
lost communication: communication that cannot be serviced as expected by the users, for reasons internal to the
service and outside of control of the users
modality: way in which communication is perceived or is expressed
NOTE: The most valid examples for the present document are signed (= using sign language), written and spoken
modalities.
primary user (of relay service): person who is using relay services because of a disability
NOTE 1: Primary users are the target users of relay services who need some communication modality conversion
or other communication support in order to communicate with voice users.
NOTE 2: A primary user can initiate and receive communications.
Real-Time Text (RTT): form of text conversation in point-to-point situations or in multipoint conferencing where the
text being entered is displayed in such a way that the communication is perceived by the user as being continuous on a
character-by-character basis
relay service: electronic communications service which enables real-time bidirectional communication between end-
users who use different communication modalities or need other communication support
NOTE: This is typically achieved by providing conversion between different communication modalities such as
text, sign, and speech, or by providing the needed cognitive support for end-users during the
communication, by a human operator or automatic means.
Relay Service Invocation Functions (RSIF): set of functions related to decisions, communication establishments and
media interconnections between users and a relay service
NOTE: The implementation of RSIF may be distributed between communications services, primary user
equipment and relay service.
relay service user: primary or secondary user of a relay service
secondary user (of relay service): communication user other than a primary user, participating in a communication
facilitated by a relay service that is invoked based on the primary user's preferences or actions
NOTE 1: A secondary user may initiate or answer a communication in which a relay service is invoked by primary
user's preferences or actions.
NOTE 2: A secondary user may also be another primary user participating in a communication via another relayed
communication.
sign language interpreter: person working in a video relay service with sign language interpretation
NOTE: Also called "interpreter".
speech to speech relay service: electronic communications service that enables electronic communication users with
speech disabilities and other users to interact by providing skilled communications assistance between them
text relay service: electronic communications service that enables users of user equipment with RTT capabilities and
users of user equipment with voice capabilities to interact by providing conversion between the two communication
modalities in substantially real time
NOTE: This conversion is provided by a CA.
total conversation: audio-visual conversation service providing bidirectional symmetric real-time transfer of motion
video, text and voice between users in two or more locations
Video Relay Service (VRS): service that enables communication between sign language users and voice
communication users by providing real-time interpretation between sign language and spoken language
NOTE: This conversion (interpretation) is normally provided by a sign language interpreter.
ETSI
10 ETSI ES 202 975 V3.1.1 (2026-05)
3.2 Symbols
For the purposes of the present document, the following visual symbols apply to indicate the three communication
media:
Table 3.1: Visual symbols used to represent communication media
Symbol Description
Audio communication
Real-time text communication
Video communication
For the purposes of the present document, the following visual symbols apply to indicate the three communication
modalities:
Table 3.2: Visual symbols used to represent communication modalities
Symbol Description
Spoken
Written
Signed
3.3 Abbreviations
For the purposes of the present document, the following abbreviations apply:
CA Communications Assistant
FCC Federal Communications Commission
GSM Global System for Mobile communication
HTML HyperText Markup Language
ICT Information and Communication Technology
IMS IP Multimedia Sub-system
IP Internet Protocol
IVR Interactive Voice Response
NPRM Notice of Proposed RuleMaking
PSAP Public Safety Answering Point
RSIF Relay Service Invocation Functions
RTT Real-Time Text
ETSI
11 ETSI ES 202 975 V3.1.1 (2026-05)
SIP Session Initiation Protocol
StT Speech-to-Text
TtS Text-to-Speech
VoIP Voice over IP
VRS Video Relay Service
WebRTC Real Time Communication in Web browser
4 General functional requirements on relay services
and use cases
4.1 Relay service types
4.1.1 General
A relay service is an ICT service, as outlined in Figure 4.1, that enables users of different communication modalities to
interact by providing conversion between the modalities.
The invocation of relay service in ICT communication is handled by Relay Service Invocation Functions (RSIF) which
may be provided separately but interoperating with the relay service.
The connections in Figure 4.1 represent general session and media connections and do not show the technological
routes of the connections or points of media mixing.
The user who makes use of a relay service because of a disability is called primary user in the present document, and
another user involved in the communication is called secondary user.
NOTE: Both primary and secondary users can initiate outgoing communications as well as receive incoming
communications where relay service is invoked.

Figure 4.1: Electronic communication supported by a relay service
EXAMPLE: The relay service is facilitated by a Communications Assistant (CA), converting (interpreting)
real-time communication between a user who has no or limited use of voice communication
(e.g. sign language user or text user) and a voice communications user. The service can be
provided by a relay service provider over any form of connection, for example over a mobile
network or via an IP connection where the text/video/audio device can be based on mainstream
ICT equipment. The relay service invocation functions connect the users and the relay service.
ETSI
12 ETSI ES 202 975 V3.1.1 (2026-05)
The aim of a relay service is to support users with disabilities in their communication with other users. The relay service
support may be provided within the same service or across interoperating services and be delivered by:
• human Communication Assistant (CA) only
• human CA supported by automated CA (for example, speech-to-text tools)
• automated CA only - at the time of writing of the present document, relay services providing text relay and
captioned telephony are available with automated CA only
The continuation of clause 4.1 specifies the recognized types of relay services. Each type makes use of a typical default
set of media connections and provides the relevant conversions or other communication support. A set of tables in
clause E.1 of the present document shows these media connections and the needed conversions or other communication
support.
4.1.2 Text relay service
A text relay service shall enable the conversion:
• from real-time text, coming from user equipment with RTT capabilities, to speech delivered to user equipment
with audio communication capabilities
• from speech, coming from user equipment with audio communication capabilities, to real-time text delivered
to user equipment with RTT capabilities
NOTE 1: The conversion between the two communication modalities may be provided by a human CA, by
automated speech-to-text/text-to-speech technologies, or semi-automated conversion where a
communication assistant is supported by automated speech-to-text/text-to-speech technologies.
The following variations from this basic operation shall be provided, either by pre-defined user preference configuration
or by requesting a variation at the beginning of the communication:
• No conversion towards the text primary user who can hear, i.e. the primary user sends text, but can hear
directly the secondary user.
NOTE 2: This variation of the text relay service is sometimes referred to as Hearing Carry Over (HCO).
• No conversion from the text primary user who can talk, i.e. the primary user talks, but the speech of the
secondary user is being converted to text for the primary user to read.
NOTE 3: This variation of the text relay service is sometimes referred to as Voice Carry Over (VCO).
4.1.3 Video relay service
A video relay service shall enable communication between sign language users and voice communication users by
providing real-time interpretation between sign language and spoken language.
NOTE 1: This conversion will normally be provided by a sign language interpreter.
The video relay service shall have the supported sign and spoken languages announced.
All communication parties (including the video relay service) shall be enabled to use RTT a
...