Which of the following is true about calls terminated by the IP phone?

A call terminated by an IP phone is signified by the transmission of a BYE message. In the context of the Session Initiation Protocol (SIP), the BYE message is specifically designed to indicate that one party has chosen to end an existing call. Once the call participant sends the BYE message, the other participant acknowledges it with an OK response, which formally concludes the call session.

Understanding the other options provides clarity on why they are not applicable in this scenario. A CANCEL message would be used to cancel a pending request for a call establishment, not to terminate an already established session. A SIP INVITE message initiates a call, not terminates it. Lastly, stating that no messages are sent is incorrect because there is indeed a structured process involving the BYE message to formally end the call. Thus, the correct statement accurately reflects the functioning of SIP in terminating a call via an IP phone.