[SIPForum-discussion] 183 no sdp and ringback tone

Tue Sep 16 14:24:30 UTC 2008

I have a question regarding ringtone and 183 without SDP.  We have a call from SIP to ISDN BRI.  Both channels are busy, but the ISDN gw sends 183 while trying the two b channels.  This is causing the SIP phone to play ringback tone to the caller.  Then the ISDN GW sends a 486 and then we get the busy tone.  Of course this is unwanted behavior.  I can't find any reference that says a UA should provide or not provide ringback tone with a 183.  Anyone care to enlighten me?

Anthony