http://www.rfc-editor.org/rfc/rfc3842.txt
RFC 3842: A Message Summary and Message Waiting Indication Event Package for the Session Initiation Protocol (SIP), R. Mahy2004-08-01T23:00:00-00:00This document describes a Session Initiation Protocol (SIP) event package to carry message waiting status and message summaries from a messaging system to an interested User Agent. [STANDARDS-TRACK]