Ignoring the frequencies for a moment, because physics, the phone does 'wi-fi' calling just fine, a 40 'channel' (or 'room') push to talk (or VOX) voice app is pretty trivial. You get close just running the discord client on your phone. That _does_ require WiFi, but _doesn't_ require a cell tower. So modally, totally doable from a WiFi network, if your making your own WiFi network then yeah, you need a cell tower.