Don't worry Viber is working in VoIP for WP8
This is the latest Answer from Viber. I got it from WPCentral forum. Wish this answer for your curious about this.
And if they complete the App, this Viber will be exclusive for Nokia users. They can use VoIP in viber. Viber is a multi platform messaging apps. VIBER now available in iPhone, Android and Windows Phone. It provides messages sending and VoIP. You can use it to send a free SMS via internet and call via internet for free. I used to use it for calling friends in Europe. It is cheaper than normal call.