What SIP response code indicates that the user is unavailable?

The response code indicating that the user is unavailable is 480 Temporarily Unavailable. This specific response is part of the Session Initiation Protocol (SIP) and is used to inform the caller that the user is currently unavailable to take the call but may be reachable at a later time. The use of "temporarily" suggests that this is not a permanent condition and that the user might be available again shortly.

In contrast, the other codes serve different purposes. For example, 404 Not Found indicates that the user’s address does not exist in the system; 403 Forbidden signifies that the server understands the request but refuses to authorize it, indicating a permission issue; and 200 OK generally confirms that a request has been successfully processed, which is not applicable in the context of unavailability.