That changes today as Viber is rolling out a massive update for Windows Phone 8 adding VoIP calling with HD audio. In addition, you get lockscreen notifications and the option to pin conversations as Live Tiles on the Start screen.
The new Viber for Windows Phone also allows sharing pictures and group chats with as many as 40 people. If you want to use the free voice calls at all times without having to worry about a running Skype app in the background, or if you just don’t like Skype for some reason, Viber is a great alternative.
source: Windows Phone Blog