This standard specifies the stage three of the Three-Party (3PTY) supplementary service for the pan-European Integrated Services Digital Network (ISDN) as provided by European public telecommunications operators at the T reference point or coincident S and T reference point (as defined in CCITT Recommendation 1.411 [1]) by means of the Digital Subscriber Signalling System No. one (DSS1) protocol. Stage three identifies the protocol procedures and switching functions needed to support a telecommunications service (see CCITT Recommendation 1.130 [2]). In addition this standard specifies the protocol requirements at the T reference point where the service is provided to the user via a private ISDN. This standard does not specify the additional protocol requirements where the service is provided to the user via a telecommunications network that is not an ISDN. The 3PTY supplementary service enables a user to establish@ participate in and control@ a three- way conversation@ i.e. a simultaneous communication involving the served user and two remote users. The 3PTY supplementary service is applicable to all circuit-switched telecommunication services carrying speech. Further part(s) of this standard shall specify the method of testing required to identify conformance to this standard. This standard is applicable to equipment@ supporting the 3PTY supplementary service@ to be attached at either side of a T reference point or coincident S and T reference point when used as an access to the public ISDN.