Number Translation in SIP URIs
Number translations only change the user portion of the URI. A typical SIP URI looks like sip:user@hostname. The user portion can take the form of either a phone number or any other string used for identification purposes.
Within the SIP header exists a Request URI, a To URI, and a From URI. The session translation element’s rules calling parameter modifies the From URI, while the rules called parameter modifies the Request URI and the To URI.