ISO/IEC 13865:2003 specifies Supplementary Service Call Transfer (SS-CT), which is applicable to various basic services supported by Private Integrated Services Networks (PISNs). Basic services are specified in ISO/IEC 11574.
SS-CT is a supplementary service which enables a user to transform two of that user's calls (of which one must be answered) into a new call between the other two users of the two calls.
Supplementary service specifications are produced in three stages, according to the method described in ETS 300 387. ISO/IEC 13865:2003 contains the stage 1 and stage 2 specifications of SS-CT. The stage 1 specification (clause 6) specifies the supplementary service as seen by users of PISNs. The stage 2 specification (clauses 7 and 8) identifies the functional entities involved in the supplementary service and the information flows between them.
ISO/IEC 13865:2003 contains two stage 2 specifications reflecting different ways of operating the service within the network: transfer by join and transfer by rerouteing.