Many of our clients are using our GSM gateways to allow for the integration of the GSM cell network to any PBX (legacy or next gen). For instance, our CGW-PX has three PRI connections and one IP connection, Cell users can access the PBX via the gateway (MTM) then get routed out via the IP connection overseas.
look for FemtoCell technology, which effectively does this. It is a local base station that then routes the calls over IP. Not too sure if it can be used by private organisations, mainly designed as another way to access the Telco’s network, but that is where you should look for a start.