http://www.rfc-editor.org/rfc/rfc3841.txt
RFC 3841: Caller Preferences for the Session Initiation Protocol (SIP), J. Rosenberg, H. Schulzrinne, P. Kyzivat2004-08-01T23:00:00-00:00This document describes a set of extensions to the Session Initiation Protocol (SIP) which allow a caller to express preferences about request handling in servers. These preferences include the ability to select which Uniform Resource Identifiers (URI) a request gets routed to, and to specify certain request handling directives in proxies and redirect servers. It does so by defining three new request header fields, Accept-Contact, Reject-Contact, and Request-Disposition, which specify the caller's preferences. [STANDARDS-TRACK]