scope:

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.