This document provides a means for a Session Initiation Protocol (SIP) User Agent (UA) that receives a dialog-forming request to supply its identity to the peer UA by means of a request in the reverse direction, and for that identity to be signed by an Authentication Service. Because of retargeting of a dialog-forming request (changing the value of the Request-URI), the UA that receives it (the User Agent Server, UAS) can have a different identity from that in the To header field. The same mechanism can be used to indicate a change of identity during a dialog, e.g., because of some action in the Public Switched Telephone Network (PSTN) behind a gateway. This document normatively updates RFC 3261 (SIP).