Critical resource data (192) required for a group call can be identified. Controlling (115) and secondary (155) PTT communications systems can communicate using an expanded network-to-network interface (NNI) (140). Allocation of identified critical resources (150) can be requested (145) for the group call from the secondary PTT communications system (155). In response to a successful allocation of the requested critical resources by the secondary PTT communications system, the group call can be authorized by the controlling PTT communications system (115). The allocated critical resources of the secondary PTT communications system (155) can be used to establish the group call.
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