ISO/IEC 15505:2003规定了补充业务消息等待指示（SS-MWI），它与但不限于专用综合业务网（PISN）支持的各种基本业务有关。ISO/IEC 11574规定了基本服务。 该补充业务MWI使得被服务用户能够被发送消息等待指示，并且还使得该消息等待指示能够被取消。 根据ETS 300 387中描述的方法，服务规范分三个阶段制定。 ISO/IEC 15505:2003包含SS-MWI的第1阶段和第2阶段规范。第一阶段规范（第6条）规定了PISNs用户看到的补充业务。第2阶段规范（第7条）规定了补充业务中涉及的功能实体以及它们之间的信息流。

ISO/IEC 15505:2003 specifies supplementary service Message Waiting Indication (SS-MWI), 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. This supplementary service MWI enables a Served User to be sent a Message Waiting Indication and also enables this Message Waiting Indication to be cancelled. Service specifications are produced in three stages, according to the method described in ETS 300 387. ISO/IEC 15505:2003 contains the stage 1 and stage 2 specifications of SS-MWI. The stage 1 specification (clause 6) specifies the supplementary service as seen by users of PISNs. The stage 2 specification (clause 7) specifies the functional entities involved in the supplementary service and the information flows between them.