SIP 400 Bad Request
What SIP 400 means
The request was malformed. Very little SIP equipment sends genuinely broken messages, so when a 400 appears the usual explanation is that something rewrote the message in transit, and SIP ALG is the prime suspect.
What SIP 400 Bad Request actually means
The request could not be understood due to malformed syntax. It is a parsing failure rather than a policy decision, so the question is not what you asked for but what actually arrived at the far end.
What causes SIP 400 on a business phone system
- SIP ALG has rewritten headers badlyA router altering SIP in flight can produce genuinely invalid messages, particularly where addresses are substituted inconsistently. This is the most common explanation on a business broadband connection.
- A provisioning template has produced an invalid fieldAn unusual character in a display name, or a field left with placeholder text, can render a header invalid.
- Firmware on the device is producing non-compliant messagesOlder or unpatched firmware occasionally sends headers a strict server rejects. Worth checking where one model of handset fails and others do not.
How to fix SIP 400
- Disable SIP ALG and retestIf the 400 disappears, you have your answer and no further diagnosis is needed.
- Check display names and provisioning fields for odd charactersLook for quotes, angle brackets and accented characters in names, and for any field still holding template placeholder text.
- Update the handset firmwareWhere a single model is affected, its firmware is a likely cause and updating is straightforward.
Fixing the underlying problem
This page explains the code. These guides walk through the fix in detail.
Questions about SIP 400
What causes SIP 400 Bad Request?
Something sent a message the receiving server could not parse. Since most SIP equipment sends valid messages, the usual cause is a router with SIP ALG enabled rewriting the message in transit and breaking it. Disable ALG before looking anywhere else.
Related SIP response codes
Source. SIP 400 Bad Request is defined in RFC 3261 section 21.4.1 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.