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How to transmit audio through FS1000a TX/RX set? (Or other alternatives like PBX?)?

I have a huge bulkload of FS1000a TX/RX sets, and I need to somehow build a local 2-way audio system. It'll be something like a group of walkie-talkies, but I can transmit audio 2 way simultaneously amongst all the sets (rather than 1 way for walkie talkie). Is there any way I can inexpensively build 4 or 5 of those?

Or, are there any alternatives? I've thought of modifying cordless phones but they can prove expensive, and I also don't have knowledge of building some sort of local-based PBX to connect all the phones together and I can dial anyone of them at will (on top of conference-calling all connected phones). Even if I can build a PBX its not that portable.