What it means
The request was malformed and the server could not understand it. Something in the SIP syntax, a required header, or the SDP is wrong.
SIP/2.0 400 Bad Request
Common causes
- A missing or malformed mandatory header such as To, From, Call-ID, CSeq or Via
- Broken or invalid SDP in the body
- A non-standard header the far end rejects
- A bug in the sending stack producing invalid SIP
How to fix it
- Capture the exact INVITE and check every mandatory header is present and well formed
- Validate the SDP offer
- Compare against a known good trace to spot the malformed field
- Update or patch the sending device if it emits invalid SIP
See it in your own trace
Paste a full SIP or Asterisk log into the decoder and ladder tools to see exactly where this response appears in the call and what led to it. Everything runs in your browser, so your logs never leave your device.
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