What response will a SIP server return if a SIP message contains an unrecognized method?

When a SIP server receives a request containing an unrecognized method, it responds with a "501 Not Implemented" status code. This response indicates that the method specified in the request is not supported by the server or is not allowed. The purpose of this status code is to inform the sender that the server recognizes the request format but does not recognize the requested method itself.

In the context of SIP, methods are specific actions defined in the SIP protocol, such as INVITE, OPTIONS, and REGISTER, among others. If a SIP message includes a method that is not defined in the SIP standards or that the server simply does not support, the server appropriately responds with the "501 Not Implemented" status to signal that it cannot fulfill the request due to this unrecognized method.

This helps maintain clear communication between SIP endpoints, as it provides feedback on the legitimacy and acceptance of the methods being employed in requests.