420 4xx Client error RFC 3261
SIP 420 Bad Extension
The server does not understand a protocol extension listed in the Proxy-Require or Require header.
Typical Q.850 / ISDN cause: 127 (Interworking, unspecified)
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Common causes
- The caller requires an extension such as 100rel, timer or replaces that the far end does not support.
How to fix it
- Move the extension from Require to Supported on the sending side, or disable it (for example, set 100rel to 'supported' rather than 'required').
Seeing 420 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.