What SIP message should you expect back from your ITSP's registration server once a SIP trunk registration has been accepted?

When a SIP trunk registration is successfully accepted by the Internet Telephony Service Provider's (ITSP) registration server, the appropriate response message is "200 OK." This status code indicates that the server has received and successfully processed the SIP REGISTER request from a client. In the context of SIP (Session Initiation Protocol), a "200 OK" response serves as confirmation that the registration has been completed successfully, allowing the client to proceed with making or receiving calls through the SIP trunk.

The other responses play different roles in SIP messaging. A "202 Accepted" message indicates that the server has accepted a request for processing but has not completed the action, which is not applicable in the case of confirmed registration. "100 Trying" is an interim response used to signal that the server is processing the request, but it does not indicate a successful registration. Lastly, "401 Unauthorized" indicates that authentication is required for the request to be completed, suggesting that the registration attempt failed due to authentication issues, not a successful reception. Thus, "200 OK" is the definitive confirmation of a successful SIP trunk registration process.