This International Standard specifies the signalling protocol for the support of the Call Completion supplementary services at the Q reference point between Private Integrated services Network exchanges (PINXs) connected together within a Private Integrated Services Network (PISN). The Call Completion supplementary services are Completion of Calls to Busy Subscribers (SS-CCBS) and Completion of Calls on No Reply (SS-CCNR). SS-CCBS enables a calling User A, encountering a busy destination User B, to have the call completed when User B becomes not busy, without having to make a new call attempt. SS-CCNR enables a calling User A, encountering a destination User B that, though alerted, does not answer, to have the call completed when User B becomes not busy again after a period of activity, without having to make a new call attempt. The Q reference point is defined in ISO/IEC 11579-1. Service specifications are produced in three stages and according to the method specified in ETS 300 387. This International Standard contains the stage 3 specification for the Q reference point and satisfies the requirements identified by the stage 1 and stage 2 specifications in ISO/IEC 13866. The signalling protocols for SS-CCBS and SS-CCNR operate on top of the signalling protocol for basic circuit switched call control, as specified in ISO/IEC 11572, and uses certain aspects of the generic procedures for the control of supplementary services specified in ISO/IEC 11582. This International Standard also specifies additional signalling protocol requirements for the support of interactions at the Q reference point between SS-CC and other supplementary services and ANFs. This International Standard is applicable to PINXs which can interconnect to form a PISN.