This International Standard specifies the Supplementary Services Do Not Disturb (SS-DND) and Do Not Disturb Override (SS-DNDO), which are applicable to various basic services supported by Private Integrated Services Networks (PISN). Basic services are specified in ISO/IEC 11574. SS-DND is a supplementary service which enables a served user to cause the PISN to reject any calls, or just those associated with a specified basic service, addressed to the served user's PISN number. The calling user is given an appropriate indication. Incoming calls are rejected as long as the service is active. The served user's outgoing service is unaffected. SS-DNDO is a supplementary service which enables a served user to override SS-DND at a called user; that is, to allow the call to proceed as if the called user had not activated SS-DND. SS-DND and SS-DNDO are described separately because SS-DND is a service used by a called user, and SS-DNDO is a service used by a calling user. This leads to describing two very related state machines. Note 1 — It is possible to implement SS-DND without implementing SS-DNDO. Supplementary service specifications are produced in three stages, according to the method described in CCITT Recommendation I.130. This International Standard contains the stage 1 and stage 2 specifications of SS-DND and SS-DNDO. The stage 1 specifications (clauses 6 and 7) specify the supplementary service as seen by users of PISNs. The stage 2 specification (clauses 8 and 9) identify the functional entities involved in the supplementary service and the information flows between them.