Viber is a new, free app for iPhone that allows you to make free phone calls anywhere in the world. Just install the app and give your phone number, that’s all – no registration required. However, you can only make free phone calls to other iPhone users that have Viber installed.
Moreover, you don’t need to add contact list to Viber, it’ll use your contact list and add a Viber logo to the contact, if Viber installed.
Besides that, you don’t even need to run the app to receive phone calls, because it has a push notification service from Apple’s servers.