A proxy server of an Internet protocol (IP) telephony system determines when a call setup message generated by a first of the IP telephony system's customers is directed to a second of the IP telephony system's customers. When that occurs, the call setup message is forwarded directly to a call cluster operated by the IP telephony system and that services the second customer. As a result, the call is handled internally by the IP telephony system, and there is no need to involve a second telephony system in conducting the requested call. The proxy server may also insert information into the call setup messaging to flag the call as an internal call between two customers of the IP telephony system.