Session Manager supports simultaneous registration of telephones, a method that provides the greatest robustness using the SIP-outbound semantics. This means that SIP telephones simultaneously register with core Session Managers and the survivable Session Manager. The telephones accept incoming calls from any of these servers and automatically perform active controller selection according to existing algorithms. This means that although SIP telephones can receive calls from any of the registered controllers, telephones initiate calls through only the highest priority controller, the active controller. With an active controller outage, telephones mark the next controller in the list as the active controller for outbound services. Upon detecting the revival of the highest priority server, telephones move back to the revived controller as the active controller.