A Session Initiation Protocol (SIP) system controls an Internet Protocol (IP) communication session between a first communication device and a second communication device. A SIP control server receives a SIP bye message from the first communication device indicating a loss of the IP communication session, and in response, transfers a notification request for the second communication device to a SIP registration server. The SIP registration server processes a SIP registration message from the second communication device, and in response to the notification request and the SIP registration message, transfers a registration notification for the second communication device to the SIP control server. The SIP control server receives the registration notification for the second communication device, and in response, transfers a SIP invite message to re-establish the IP communication session to the first communication device.