[codetrotter]

What kind of implications do you mean? And what type of free calls?

If you only mean calling other people using the app then WhatsApp, Facebook Messenger and a bazillion other apps already do this.

[01635]

An app that is not bound to its own users. You could make free call to any number, in any country. I assume, telecom giants will not entertain such idea. Rebtel allow users to use 3 minutes free time to make calls anywhere (even to people who do not have internet connection). Imagine, if you could have a feature in Signal, where you make calls to active phone numbers outside your peer-to-peer Signal contacts. Are there any negative implications of it?

[codetrotter]

How would the company behind this app pay for all of that phone traffic?

[01635]

I agree, that's a massive hurdle.