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?