Can anyone suggest an offline local voice chat app for Android and iOS?

It really seems like I ought to be able to connect a few devices locally via Bluetooth or teathering, and be able to have an open voice chat between those devices without an internet connection, and without push-to-talk.

Anyone aware of something like this that works? I've tried a few candidates, but I haven't had much luck getting something to work.

For extra context on why I want this: I lead a sport kite team, and as the leader, I give vocal commands to tell the rest of the team what to do, and keep us coordinated. We usually use walkie talkies for this, but the one-way communication, the limited non-PTT support, and potential international regulation problems make this workable but not ideal.