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 operator at the T reference point or coincident S and T reference point (as defined in CCITT Recommendation I.411 [1] by means of the Digital Subscriber Signalling System No. one (DSS1) protocol. State three identifies the protocol procedures and switching functions needed to support a telecommunications service (see CCITT Recommendation I.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 a three party conversation@ i.e. a simultaneous communication between three parties. The 3PTY supplementary service is applicable to all telecommunication services carrying speech. It is not applicable to non-voice services. 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.