What response does a SIP client receive when an incorrect logon or password is entered?

When a SIP client enters an incorrect logon or password, the most appropriate response it receives is a 401 - Unauthorized. This status code indicates that the request has not been applied because it lacks valid authentication credentials for the target resource. In other words, the SIP server is notifying the client that the credentials provided are not adequate to gain access, prompting the client to retry with correct login information.

The 401 - Unauthorized response is critical in SIP as it not only informs the client of the authentication failure but also indicates that re-authentication is possible. Typically, the response includes a 'WWW-Authenticate' header, which specifies the authentication method that should be used for re-attempting the request.

Understanding the context of the other status codes adds clarity: a 200 - OK indicates successful request processing, a 403 - Forbidden means that access is denied regardless of authentication, and a 404 - Not Found signifies that the requested resource could not be found, which are not relevant to authentication failures. Thus, 401 - Unauthorized is the correct response in the scenario where login credentials are incorrect.