421 4xx Client error RFC 3261
SIP 421 Extension Required
The server needs a particular extension, listed in the Require header of the response, to process the request.
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Common causes
- The far end requires 100rel (PRACK) or session timers, but the caller did not offer them.
How to fix it
- Enable the listed extension on the caller (for example, PRACK support).
Seeing 421 on calls through a router or firewall? Check that SIP ALG is disabled. It causes a surprising number of SIP errors. Read why.
About 4xx responses
The request has a problem or cannot be completed at this server, usually because of something on the requesting side or the target user.