The present application describes various embodiments that address the need in wireless environments and within the SIP framework to efficiently maintain SIP contact addresses. It introduces the concept of a SIP proxy user agent (UA) (e.g., 123 or 124) to serve as a gateway between a SIP core network and a SIP-unaware mobile (101). A new parameter, called "created," is described for the contact header in SIP 200 OK messages, which is used to trigger deregistrations by SIP proxy UAs that no longer serve a particular mobile. It is this process for deregistering SIP proxy UAs that allows SIP contact addresses to be more efficiently maintained.