Patent ID: 8693464

Claim:
An interrogating call session control function server operating in an internet protocol multimedia subsystem core of an internet protocol multimedia subsystem, the server comprising: a memory to store computer instructions; and a controller coupled to the memory, wherein the controller, responsive to executing the computer instructions, performs operations comprising: receiving a request for a voice call associated with an originating communication device operating in a first network; routing the request to a first server of the internet protocol multimedia subsystem for facilitating establishment of the voice call when an internet protocol address is determined to be available for establishing the voice call with a recipient communication device; and routing the request to a second server of a second network for facilitating establishment of the voice call without routing the request to another server of the internet protocol multimedia subsystem when the internet protocol address is determined not to be available for establishing the voice call with the recipient communication device, wherein the interrogating call session control function server is selected from a group of interrogating call session control function servers based on a local routing number being hosted by the interrogating call session control function server, wherein call routing decisions associated with the internet protocol multimedia subsystem and the second network are monitored, and wherein a list of candidate end user devices is generated for migration to the internet protocol multimedia subsystem.