Viber is an iPhone application (Android and Blackberry versions coming soon!) that lets you make free phone calls to other iPhone users that have Viber installed.
When you use Viber, your phone calls to any other Viber user are free, and the sound quality is much better than a regular call. You can call any Viber user, anywhere in the world, for free. All Viber features are 100% FREE and do not require any additional “in application” purchase.
How does it work and why is better than skype (in the case that you are doing iPhone to iPhone calls)?
You don’t need to have a login name. Just your telephone numbers. It scans your address book and find those with viber installed, then you can call them free, well, using part of your data.
Works via wifi or 3g or even gprs. Give it a try!