The service uses credit that can be purchased through in-app purchases, or via credit card, and the minutes can be used on Android, iOS or your desktop. It’s actually a lot like how Skype works, except it looks like Viber’s rates are much lower and it won’t charge connection fees…
From Viber’s press release (via Android Central):
“Now with the official worldwide launch, Viber Out enables all Viber users to call anyone worldwide at low rates on Android, iPhone and Desktop, coming to Windows Phone in the future. Viber Out credit can be bought with an in-app purchase from the Google Play Store or the Apple App Store, or with a credit card via Viber Desktop. Additionally, Viber Out has a significantly lower price-per-call than other competing services due to lower per minute rates and the omission of connection fees. […]
Unlike competing services, Viber Out displays your real phone number to the recipient so he or she knows who is calling. Further, Viber Out on Desktop has your phone’s address book so you don’t have to remember numbers or dial them manually.â€
You can download the new update from the App Store