SIP 183 Session Progress
What SIP 183 means
Audio is being sent before the call is answered. This is how you hear a network announcement, a carrier ringing tone or a message saying the number is unavailable, and it is not a fault.
What SIP 183 Session Progress actually means
The call is progressing and information about it is being conveyed, typically as early media carried in the session before any final response. It is what allows a network to play you an announcement instead of leaving you listening to silence.
What causes SIP 183 on a business phone system
- The carrier is playing an announcementMessages such as a number being unavailable or a call not being connected arrive as early media, which is why you hear a recorded voice rather than a tone.
- Ringing tone is generated by the networkWhere the network rather than your own system generates the ringing tone, it arrives this way.
- Early media is being used for progress on international callsLong-distance and mobile destinations commonly signal progress with early media, which can be mistaken for the call having connected.
How to fix SIP 183
- Treat it as information, not a faultA 183 in a log tells you the call was progressing normally.
- Listen to the announcement to find the real causeWhere callers report a strange message, the announcement carried in the early media usually states the actual problem in plain English, which is far more useful than the code.
- Check early media handling if audio starts lateWhere a system suppresses early media, callers hear silence during setup and assume the call has failed. Enabling it removes a common complaint.
Fixing the underlying problem
This page explains the code. These guides walk through the fix in detail.
Questions about SIP 183
What does SIP 183 Session Progress mean?
The call is on its way and the network is sending audio before it is answered. That early audio is how you hear ringing tone or a recorded announcement, so a 183 is a sign of normal progress rather than a fault.
Why do I hear a recorded message instead of ringing?
Because the network is using early media to play you an announcement. Listen to what it says: the recording usually explains the real problem, such as the number being unavailable, far more directly than any status code will.
Related SIP response codes
Source. SIP 183 Session Progress is defined in RFC 3261 section 21.1.5 and registered in the IANA SIP Parameters — Response Codes. The causes and fixes above are drawn from our own experience supporting UK business phone systems.