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.