Abstract: A network-to-network interface (NNI) gateway for inter-communication between a Push-to-Talk-over-Cellular (PoC) system in a first wireless network and a Land Mobile Radio (LMR) system in a second wireless network. The PoC system performs a PoC call session for PoC mobile units in the first wireless network. The LMR system performs a Push-to-Talk (PTT) call session for LMR subscriber units in the second wireless network. Both the PoC and PIT call sessions comprise an instant two-way half-duplex voice call within a group of the PoC mobile units and/or LMR subscriber units. The gateway bridges the LMR system to the PoC system, such that the PoC system is exposed to the LMR system as an emulated LMR system, the LMR system is exposed to the PoC system as an emulated PoC system, and calls are placed across the first and second wireless networks between the PoC mobile units and LMR subscriber units.