said by Stewart:For 300 yards, you may need quite a few access points, depending on the site (open field, factory floor, warehouse, hotel, etc.) The system may need careful setup to avoid audio gaps or dropped calls when a user roams from one AP to another.
If you already have Wi-Fi installed in the target area, set up an app on a smartphone and walk around while talking. For this purpose, you could use a SIP app with a commercial VoIP provider, though a quick test with Skype or Hangouts should give similar results. Also, run a speed test in several locations -- you'll need ~1 Mbps up and down to handle 12 users on the same AP.
Upon reflection it's probably more like 200 yards (largely outdoors; we'd even have line-of-sight most of the time) and it's not omni-directional. Basically it would be linking two groups of people and while the groups might be as far as 200 yards apart (and sometimes barely 50) the people within the group would be relatively close together.
Skype won't work because we're not likely to have Internet access (and if we did it would rarely be fast Internet access). That's why I was hoping for some sort of self-contained VOIP server that would run on a laptop or Raspberry Pi. These groups only need to communicate with each other at the site.