Unregister SIP UAC message

An "unregister" is a REGISTER wherein you set the expires for one or more Contact URIs to 0. You may do so either with the Expires header - Expires: 0 - which applies to all Contact URIs in the REGISTER, or with a parameter - Contact: <sip:foo@bar>;expires=0 - which will only affect that URI's registration.

You may also unregister ALL contact URIs for your address-of-record by adding Contact: * and Expires: 0.