183 1xx Provisional RFC 3261
SIP 183 Session Progress
Carries progress information, usually with an SDP body for early media, so the caller hears ringback, announcements or IVR audio before the call is answered.
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Common causes
- Carrier announcements, network ringback, IVR prompts before answer.
How to fix it
- No audio during 183? Check that RTP from the far end reaches the caller (NAT and firewall), and that the SDP connection address is reachable.
- If early media is unwanted, convert 183 to 180 on your SBC.
Seeing 183 on calls through a router or firewall? Check that SIP ALG is disabled. It causes a surprising number of SIP errors. Read why.
About 1xx responses
The request was received and is being processed. The call is not finished yet.