This Recommendation defines the operation of the Digital Subscriber Signalling System No. 1 (DSS 1) for the support of the Three-Party (3PTY) supplementary service at the T or the coincident S and T reference point of the Integrated Services Digital Network (ISDN).
The Three-Party supplementary service allows the user to establish, participate in, and control a three‑way conversation, i.e. simultaneous communication involving the served user and two remote users.