A method (600,700) and apparatus (800) are arranged and operate for facilitating call routing between sites in an enterprise connected by a private IP network (130). A range of identifiers (220-223) served by IP PBXs (120-123) in the enterprise may be registered with a SIP registrar (212) of an inter-site router (210). Call requests to a communication unit (350) outside the home site are routed to the SIP registrar. A site identifier associated with the destination IP PBX allows the call to be completed by the SIP Proxy or the IP PBX where the call was received. When no private IP network is available, a roaming CU registers with a cellular proxy in the visited network. Calls are routed by when an HLR (340) associated with the CU obtains the roaming number from the cellular proxy (330) in the visited site. The MSC completes the call to the visited IP PBX using the roaming number and a local SIP registrar 440 forwards the call to the CU.