ISO/IEC 20116:2004 specifies supplementary service Message Centre Monitoring/Mailbox Identification (SS-MCM/MID), which is related, but not limited, to various basic services supported by Private Integrated Services Networks (PISNs). Basic services are specified in ISO/IEC 11574.

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 the event that 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 Message Centre.

Service specifications are produced in three stages, according to the method described in ETS 300 387. ISO/IEC 20116:2004 contains the stage 1 and stage 2 specifications of SS-MCM/MID. The stage 1 specification specifies the supplementary service as seen by users of PISNs. The stage 2 specification specifies the functional entities involved in the supplementary service and the information flows between them.