This International Standard specifies the signalling protocol for the support of the Do Not Disturb and Do Not Disturb Override supplementary services (SS-DND and SS-DNDO) at the Q reference point between Private Integrated services Network eXchanges (PINXs) connected together within a Private Integrated Services Network (PISN). 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 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 number; that is, to allow the call to proceed as if the called user had not activated SS-DND. 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 14842. The signalling protocols for SS-DND(O) operate on top of the signalling protocol for basic circuit switched call control, as specified in ISO/IEC 11572, and use 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-DND and other supplementary services and ANFs and between SS-DNDO and other supplementary services and ANFs. NOTE — Additional interactions that have no impact on the signalling protocol at the Q reference point can be found in the relevant stage 1 specifications. This International Standard is applicable to PINXs which can interconnect to form a PISN.