What does the SIP response code 404 indicate?

The SIP response code 404 indicates "Not Found." This response is sent when the request could not be fulfilled because the requested resource, such as a user or session identified by the Request-URI, does not exist within the domain of the server. In the context of SIP, this means that when a user attempts to connect to a specific endpoint or service, the server is unable to locate it. This can occur when the user ID is incorrect, the user is not registered, or when the requested resource is not currently available on the SIP network.

Other options reflect different issues with requests: a request timeout indicates that a request took longer than expected to receive a response; a bad request signifies an improperly formed request that the server cannot understand; and unauthorized refers to issues where valid credentials are required but not provided. Each of these codes serves a distinct purpose, pinpointing specific problems in SIP communication, whereas 404 directly pertains to the absence of the requested resource.