http://www.rfc-editor.org/rfc/rfc3578.txt
RFC 3578: Mapping of Integrated Services Digital Network (ISDN) User Part (ISUP) Overlap Signalling to the Session Initiation Protocol (SIP), G. Camarillo, A. B. Roach, J. Peterson, L. Ong2003-08-01T23:00:00-00:00This document describes a way to map Integrated Services Digital Network User Part (ISUP) overlap signalling to Session Initiation Protocol (SIP). This mechanism might be implemented when using SIP in an environment where part of the call involves interworking with the Public Switched Telephone Network (PSTN). [STANDARDS-TRACK]