REGISTER requests: REGISTER requests MAY contain a Contact header
        field indicating at which locations the user is reachable. The
        REGISTER request defines a wildcard Contact field, "*", which
        MUST only be used with Expires: 0 to remove all registrations
        for a particular user. An optional "expires" parameter indicates
        the desired expiration time of the registration. If a Contact
        entry does not have an "expires" parameter, the Expires header
        field is used as the default value. If neither of these
        mechanisms is used, SIP URIs are assumed to expire after one
        hour. Other URI schemes have no expiration times.

   REGISTER 2xx responses: A REGISTER response MAY return all locations
        at which the user is currently reachable.  An optional "expires"
        parameter indicates the expiration time of the registration. If
        a Contact entry does not have an "expires" parameter, the value
        of the Expires header field indicates the expiration time. If
        neither mechanism is used, the expiration time specified in the
        request, explicitly or by default, is used.