There is a free app called "Free calls with Magic Jack" you can find it by searching for "magic jack". It is available for both iPhones and Androids. You can make free calls to any US or Canadian numbers, or to any magic jack numbers.

It's use is pretty simple. It asks if you already have a magic jack account, you can select "no" and you are ready to use the phone. The call quality is better than with viber.