This International Standard specifies the signalling protocol for the support of the Message Centre Monitoring supplementary service (SS-MCM) as well as the Mailbox Identification supplementary service (SS-MID) at the Q reference point between Private Integrated services Network eXchanges (PINXs) connected together within a Private Integrated Services Network (PISN). The supplementary service MCM enables a Served User to get informed by a Message Centre about the status and status changes of messages stored in that Served Users Mailbox. The supplementary service MID enables a Message Centre to identify a specific mailbox of a Served User in case the Served User has more than one mailbox within the Message Centre. In addition SS-MID enables a Served User to authenticate himself/herself at a specific mailbox located within the Messages Centre. 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 20116. The signalling protocol for SS-MCM and SS-MID 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-MCM as well as SS-MID 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.