It uses cellular service or Wi-Fi, so you choose which works best for you. Although you can make calls only to existing Viber users, the app boasts 40 million users.
Another app that's great for free calls is Skype. Unlike Viber, you can place voice calls to people who don't use Skype, but there will be a small fee.
The app works just like it would on your computer. In fact, you can place a call to a computer - not just other smartphones - with the Skype app. If you don't feel like talking, Skype offers instant messaging features at no charge.
One thing Skype can't do on mobile devices is video conferences. That's where Fring steps in - it allows you to host a video chat with multiple friends at no charge!