It is definitely a gray area. We've wandered through many providers, and while some have ToS against missed calls, the provider we're currently using doesn't.

It is definitely a gray area. We've wandered through many providers, and while some have ToS against missed calls, the provider we're currently using doesn't. And we just reached a partnership with a local provider to whom we'll pay the residual costs of missed calls, and they'll guarantee us the lines. For them, it's business, and for us, it's not 100% free but much cheaper than regular calls. Regarding the walkie talkie app, yes, that's our goal :) However, one of the parties will need to have an internet connection to send the message. Receiving it is free though.