SIP 420 Bad Extension
Your device insisted on a SIP extension the far end does not understand. It is a negotiation mismatch rather than a fault, and the response names the extension at issue so you know exactly what to disable.
What SIP 420 Bad Extension actually means
The server did not understand a protocol extension the request listed as required, and it returns an Unsupported header naming the ones it cannot handle. That header is the actionable part: it removes any guesswork about which feature to turn off.
What causes SIP 420 on a business phone system
- Session timers are required but not supportedA device configured to insist on session timers against a platform that does not implement them gets refused rather than downgraded.
- Reliable provisional responses are requiredInsisting on acknowledged provisional responses against equipment that does not support them produces this, often on calls to older gateways.
- Outbound registration handling is requiredWhere a device requires the modern registration flow mechanism and the server predates it, the requirement is rejected.
How to fix SIP 420
- Read the Unsupported header from the responseIt names the exact extension the far end cannot handle, which turns this from guesswork into a single setting change.
- Change the extension from required to supportedMost devices let you offer a capability without insisting on it. Offering it lets the far end decline gracefully instead of failing the request.
- Disable the extension for that trunk onlyKeep it on internally where it works, and relax it only towards the endpoint that cannot cope.
Fixing the underlying problem
This page explains the code. These guides walk through the fix in detail.
Questions about SIP 420
How do I fix SIP 420 Bad Extension?
Related SIP response codes
Source. SIP 420 Bad Extension is defined in RFC 3261 section 21.4.15 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.