183 SIP MESSAGE- Session Progress Method

There are instances, most notably dealing with SIP to PSTN interwork-
   ing, that necessitate that the SIP called User Agent (UA) be able to
   suppress local alerting by the SIP calling UA and to set up a prelim-
   inary media session from the called UA to the calling UA.  This would
   allow the called UA to play back media prior to the full SIP session
   being set up.  This media would be used to report on the status of
   the session setup request.  It could also be used to play music while
   the session setup is attempted.  This would be useful for find-me
   like services that involve attempting multiple locations for a single
   setup request.

   The only method in the current SIP specification that allows the
   called UA to playback media would be to set up a full SIP session. In
   PSTN interworking situations (and likely in end-to-end SIP sessions)
   this will cause a billing relationship to be established between net-
   works for the session.  This causes a problem when the reason for
   setting up the media session is to indicate a failure in the session
   setup.