If you're calling a friend that live in of our 48 Rebtel countries you can make a Smart Call. All you have to do is call your friend as you would usually do and when they (hopefully) pick up, you simply ask to call you back on the number displayed on their screen.
Now the call is instantly reconnected and you and your friend can talk for as long as you want - for free. You can't put a price on that, can you? :)