This first part of ETS 300 668 specifies the stage three of the User-to-User Signalling (UUS) supplementary service for the pan-European Broadband Integrated Services Digital Network (B-ISDN) as provided by European public telecommunications operators at the TB reference point or coincident SB and TB reference point (as defined in ITU-T Recommendation I.413 [1]) by means of the Digital Subscriber Signalling System No. two (DSS2) protocol. Stage three identifies the protocol and procedures and switching functions needed to support a telecommunication service (see ITU-T Recommendation I.130 [2]). In addition@ this ETS specifies the protocol requirements at the TB reference point where the service is provided to the user via a private B-ISDN. This ETS does not specify the additional protocol requirements where the service is provided to the user via a telecommunications network that is not a B-ISDN. The UUS supplementary service enables a user to send eceive a limited amount of information to/from another user over the signalling virtual channel in association with a call to the other user. The UUS supplementary service is applicable to all telecommunication services. Further parts of this ETS specify the method of testing required to identify conformance to this ETS. This ETS is applicable to equipment@ supporting the UUS supplementary service@ to be attached at either side of a TB reference point or coincident SB and TB reference point when used as an access to the public B-ISDN.