Viber has updated their Windows Phone messaging app to version 2.1. Unfortunately despite the tantalizing Call button the phone still does not support voice calling, merely popping up a message that the feature is not present in the beta version but that they are working to implement it.
HD Voice calling has been teased earlier, with a TechCrunch article which has since disappeared.
The biggest change seems to be an update to the name from Viber to Viber Messenger (accurately reflecting the lack of voice calls I think) and a new live tile.
The result is many dissatisfied Windows Phone users, but if you simply want to message your Viber network, the app is free and can be found in Marketplace here.