Totally free. You download it then get anyone your 3G bricks might want to talk to get it on theirs. They download to theirs themselves, or you send them an invite and they accept. Then no need for a 4G+. Video calls and all. And you can call anywhere in the world -- for free.
It was Plato who said, “He, O men, is the wisest, who like Socrates, knows that his wisdom is in truth worth nothing”