How to make free mobile call's from internet?

How does Bolt (Bolt: Free calling made simple) make free calls without using the internet?

• I've just discovered this amazing app (as matter of fact the idea seems to be), i did not have the chance to use it since i'm only using iPhone right now, but after reading almost everything from their website i could not figure it out how exactly this will work ?

• Answer:

  Bolt/Charge Talk just replaces your smartphone's default dialer, places on-net Bolt user-to-Bolt user calls over VoIP (either SIP or XMPP, like WhatsApp or Viber), and places off-net Bolt-to-PSTN calls over the public switched telephone network using your cellular operator's network. (Shazam! Pizzazz!) So Bolt-to-Bolt calls in fact would go over the public Internet.