A call session control function (CSCF) controls a communication session between a first communication device and a second communication device. The first communication device detects a loss of signal and transmits a modified session initiation protocol (SIP) bye message. The modified SIP bye message includes a code that indicates a loss of the communication session and an identifier for the second communication device. The CSCF receives the modified SIP bye message and in response to the code, subscribes with a home subscriber server (HSS) for a registration notification for the second communication device. The second communication device subsequently transfers a SIP registration request and the CSCF responsively receives the registration notification for the second communication device from the HSS that indicates a network address for the second communication device. In response to the registration notification, the CSCF transfers SIP invite messages to the first and second communication devices to re-establish the communication session.