This International Standard defines the signalling procedures and protocol for the purpose of circuit-switched Call Control at the Q-reference point between Private Integrated Network Exchanges (PINXs) connected together within a Private Integrated Services Network (PISN). The Q reference point is defined in ISO/IEC 11579-1. This International Standard is based upon that described in ITU-T Recommendation Q.931, including the provisions for symmetrical operation described in annex D of that recommendation. 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 ECMA-142, ECMA-148 and ISO/IEC 11584. This International Standard is applicable to PINXs which interconnect to form a PISN. Annex ZC is an integral part of this International Standard.