501
SIP
501 Not Implemented
The server does not support the functionality required to fulfill the request. The method is recognized but not implemented.
Когда вы видите этот код
When the SIP method is valid but the server has not implemented it (e.g., INFO, UPDATE, PRACK on a basic server).
Как исправить
Use a different method that the server supports. Check the Allow header in responses to see supported methods.