A method for ensuring critical resource availability in a push-to-talk "PTT" communications environment comprising: identifying critical resources required for a group call that belong to a secondary PTT communications system by a controlling PTT communications system that is a home system for a subscriber group to which an originating PTT subscriber unit of the group call belongs, wherein the controlling and secondary PTT communications systems are members of a PTT communications environment, wherein the controlling and secondary PTT communications systems communicate using an expanded network-to-network interface 'NNI', and wherein said expanded NNI represents a NNI that has been modified to encompass messaging and procedural elements supporting management of critical resource availability, requesting allocation of the identified critical resources for the group call from the secondary PTT communications system using the expanded NNI; wherein critical resource data corresponding to the identified critical resources is included in the request, and the critical resource data in the request is configured to specify subscriber units deemed critical with regard to participation in the group call by indicating at least one of a list of devices, a list of people or users, a role of the people, a location, an area, all users or devices within an area, or all users or devices within a radius of the originator; receiving, by said controlling PTT communications system from said secondary PTT communications system, an allocation response configured to indicate an allocation status for each identified critical resource; in response to a successful allocation of the identified critical resources by the secondary PTT communications system, authorizing the group call by the controlling PTT communications system, wherein the allocated critical resources of the secondary PTT communications system are used in establishing the group call; wherein the allocation of critical resources for the group call comprises determining, by the secondary PTT communications system, if a participating subscriber unit deemed to be critical is available or not. identifying critical resources required for a group call that belong to a secondary PTT communications system by a controlling PTT communications system that is a home system for a subscriber group to which an originating PTT subscriber unit of the group call belongs, wherein the controlling and secondary PTT communications systems are members of a PTT communications environment, wherein the controlling and secondary PTT communications systems communicate using an expanded network-to-network interface 'NNI', and wherein said expanded NNI represents a NNI that has been modified to encompass messaging and procedural elements supporting management of critical resource availability, requesting allocation of the identified critical resources for the group call from the secondary PTT communications system using the expanded NNI; wherein critical resource data corresponding to the identified critical resources is included in the request, and the critical resource data in the request is configured to specify subscriber units deemed critical with regard to participation in the group call by indicating at least one of a list of devices, a list of people or users, a role of the people, a location, an area, all users or devices within an area, or all users or devices within a radius of the originator; receiving, by said controlling PTT communications system from said secondary PTT communications system, an allocation response configured to indicate an allocation status for each identified critical resource; in response to a successful allocation of the identified critical resources by the secondary PTT communications system, authorizing the group call by the controlling PTT communications system, wherein the allocated critical resources of the secondary PTT communications system are used in establishing the group call; wherein the allocation of critical resources for the group call comprises determining, by the secondary PTT communications system, if a participating subscriber unit deemed to be critical is available or not.