As many may know, the reason why Windows Phone (before Mango) has no VOIP apps (Skype or Viber) is because Microsoft restricted it. Now, with Mango and some help from Microsoft, Viber is able to tweet about their product on Windows Phone in 2012.
Here is the link (with video) on how Viber works. Microsoft is slow on the game. Now, with Mango, I hope they can catch up. Want to know the story on how Viber team works with Microsoft? Head down to WPCentral for more details.