A communications system includes a first PTT network, and a second PTT network having a plurality of interoperability gateways. A method for routing a PTT call terminated at the second PTT network includes receiving a PTT call request from the first PTT network, the PTT call request identifying a target subscriber unit on the second PTT network and forwarding the PTT call request to a first interoperability gateway. If the target is located in a region served by a second PTT interoperability gateway, then a redirect message, including an identifier of the second PTT interoperability gateway, is received and the PTT call request is redirected to the second PTT interoperability gateway. An identifier of the target and an identifier of the second PTT interoperability gateway are stored in a cache for directing subsequent PTT calls to the same target to the corresponding second interoperability gateway. In one embodiment, the first PTT network is an iDEN network and the second PTT network is a HPPTT network and the PTT call is a HPPTT call.