3.2.4.4 Specifying Referee Identity in the Referred-By Header Field in Forwarded/Retargeted Calls

When a dialog-establishing request is forwarded, or retargeted, from one user or phone number to another, the address-of-record in the URI of the To header field in mid-dialog requests might still reflect the SIP identity of the original user or phone number, before forwarding or retargeting. If such a mid-dialog request, with the original address-of-record or phone number in the URI of the To header field, is a REFER request and the protocol client sending it intends to communicate to the SIP server the address-of-record that represents the identity of the actual message recipient after forwarding or retargeting, it SHOULD insert the ms-referee-uri parameter with the value of the SIP URI representing the address-of-record of the actual message recipient into the Referred-By header field in the REFER request. See section 2.2.5 for the specifications of the ms-referee-uri parameter.