SIP 183 Session Progress
Progress information, often with early media such as ringback or an announcement.
1xx Provisional · RFC 3261 · Answer to OPTIONS: Not a final answer. The check waits for the final response.
What SIP 183 means
183 Session Progress is a provisional response that says the call is still being set up and there is information to pass back. In practice it nearly always carries SDP, which opens early media: the caller hears audio from the far side before the call is answered. That audio can be ringback, music or a recorded announcement.
Carriers use 183 to play network announcements, and some of them are failures in disguise. A "number not in service" message can play over early media and be followed by a final error such as 404 or 480. A call that never gets past 183 has not been answered, however long the audio plays.
Example response
SIP/2.0 183 Session Progress
Via: SIP/2.0/UDP 198.51.100.12:5060;branch=z9hG4bK-183-d4
From: <sip:+[email protected]>;tag=be07
To: <sip:+[email protected]>;tag=gw-5521
Call-ID: [email protected]
CSeq: 5 INVITE
Contact: <sip:203.0.113.41:5060>
Content-Type: application/sdp
Content-Length: 214Common causes of SIP 183
- The carrier plays ringback tone as early media instead of sending a plain 180.
- The destination network plays an announcement, such as a busy or unreachable message, before it sends a final error.
- An IVR or queue plays music or a message before it answers the call.
- Early media is one-way because the SDP in the 183 advertises an address the caller can't reach, often a private IP behind NAT.
- The caller and the far end disagree on the codec, so early media arrives but can't be decoded.
How to fix SIP 183
- Listen to or record the early media: an announcement during 183 often says exactly why the call is about to fail.
- Check the connection address in the 183's SDP and confirm that RTP from that address reaches the caller.
- Compare the codec in the 183's SDP with what the caller offered; an answer outside the offer is a far-end bug.
- Look at the final response that follows the 183. A 183 followed by 404 or 480 is a failure, whatever the caller heard.
- If your SBC drops early media by policy, callers hear silence until answer; allow it, or have the SBC generate ringback.
SIP 183 as an answer to OPTIONS
OPTIONS never produces early media, so a 183 to OPTIONS isn't expected. If one arrived, PingVo would keep waiting for the final response, and a probe with nothing after the 183 counts as no answer. A healthy OPTIONS answer is final: usually 200, or an error that still proves the server is processing SIP.
Questions about SIP 183
Is 183 Session Progress an error?
No. It is a provisional response that usually means early media is starting. Whether the call succeeds depends on the final response that comes after it.
Why do I hear an announcement but the call shows as failed?
The announcement played as early media during the 183, and the call then ended with a final error. The caller heard audio, but the call was never answered.
Why is there no audio during 183?
Usually the early media can't reach the caller: a private address in the SDP, a firewall that only opens RTP after answer, or an SBC that blocks early media.