http://www.rfc-editor.org/rfc/rfc6910.txt
RFC 6910: Completion of Calls for the Session Initiation Protocol (SIP), D. Worley, M. Huelsemann, R. Jesske, D. Alexeitsev2013-04-01T23:00:00-00:00The architecture is designed to interoperate well with existing completion of calls solutions in other networks.