501 5xx Server error RFC 3261
SIP 501 Not Implemented
The server does not support the functionality needed, typically an unknown method.
Typical Q.850 / ISDN cause: 79 (Service or option not implemented)
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Common causes
- A method such as UPDATE, INFO, PRACK or MESSAGE that the server doesn't implement.
How to fix it
- Disable the feature on the requesting side, or use an alternative method.
Seeing 501 on calls through a router or firewall? Check that SIP ALG is disabled. It causes a surprising number of SIP errors. Read why.
About 5xx responses
The server failed to handle an apparently valid request.