What is the SIP response code for a successful call establishment?

The SIP response code for a successful call establishment is 200 OK. This code indicates that the request has been successfully processed and the corresponding session is established. In the context of SIP (Session Initiation Protocol), when a call is initiated and the call setup is successful, the responding user agent sends a 200 OK response back to the caller. This response not only confirms that the call has been established, but it can also include critical information regarding the session parameters, such as the media formats supported for the communication.

In contrast, the other response codes serve different purposes: 404 Not Found indicates that the requested resource could not be found, 180 Ringing is an interim response indicating that the call is being alerted (but not yet connected), and 503 Service Unavailable indicates that the server is currently unable to handle the request, possibly due to maintenance or overload. Each of these codes conveys distinct states in the call setup process, but none indicate a successful establishment of a call as effectively as 200 OK does.