Locate the response family
- 1xx ProvisionalRequest is progressing
- 2xx SuccessRequest succeeded
- 3xx RedirectionAnother destination
- 4xx Request failureInterpret this request400 belongs here
- 5xx Server failureServer could not fulfil
- 6xx Global failureFailure across destinations
Read the status with its request method and call leg. For example, 200 after REGISTER confirms a binding; it does not prove an INVITE succeeded.
What this response means
The recipient cannot process the request because it considers the message malformed. The code alone does not identify the exact invalid field.
Read it in context
Incorrect syntax, incompatible message construction or a damaged intermediary transformation can be involved. A valid-looking display name is not enough to establish that the entire SIP message is well formed.
Administrator checks
Identify the rejecting system and compare its diagnostic details with the complete sanitized request. Check URI formatting, required headers, body length and any recent proxy or device change. Compare a working request from the same path.
Next action and common mistake
Do not reset passwords as the universal remedy for malformed requests. Correct the demonstrated syntax or supported implementation mismatch and retest. Avoid publishing raw captures with identities and authentication material while seeking help.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.