In order to use it, of course, you will need to have an iPhone (it does not work with the iPod Touch or the iPad) and the contact that you wish to call will also need to have the application installed on their phone. If they do not, you will not be able to contact them.
Once you install the application on your phone the only thing you have to do is allow it access to your agenda and, in a matter of seconds, you will be able to make telephone calls with pretty good audio quality and without paying anything beyond what you pay for your internet connection.
Viber - Free Phone Calls is a very useful application that will allow its users to save a lot of money on phone calls.
Compatible only with the iPhone