This International Standard, specifies supplementary services Call Completion (SS-CC), which are applicable to various basic services supported by Private Integrated Services Network (PISN). Basic services are specified in ISO/IEC 11574. SS-CC consists of two Supplementary services: the Completion of Calls to Busy Subscribers supplementary service (SS-CCBS) and the Completion of Calls on No Reply supplementary service (SS-CCNR). SS-CCBS allows completion of a call to a subscriber that was unsuccessful because of a busy condition and SS-CCNR allows completion of a call to a subscriber that was unsuccessful because the subscriber, although alerted, did not answer. Supplementary service specifications are produced in three stages, according to the method described in CCITT Recommendation I.130. This International Standard specifies the stage 1 and stage 2 specifications of SS-CC. The stage 1 specifications (clauses 6 and 7) specify the supplementary services as seen by the users of PISNs. The stage 2 specification (clause 8) specifies the functional entities involved in the supplementary services and the information flows between them.