(...) Each proxy uses the Via header field to
   determine where to send the response and removes its own address from
   the top.  As a result, although DNS and location service lookups were
   required to route the initial INVITE, the 180 (Ringing) response can
   be returned to the caller without lookups or without state being
   maintained in the proxies.  This also has the desirable property that
   each proxy that sees the INVITE will also see all responses to the
   INVITE.