Free calls, texts, and photo messaging help make Viber one of the most popular VoIP apps in the world. Today, a new and improved version for Windows Phone 8 arrived in the Store. Download it here. The redesigned app offers free calls with HD sound, pinning of conversations to Start via Live Tiles, lock screen notifications, and beautiful improvements to its look and performance for Windows Phone 8. Connect in group conversations with up to 40 people. The app, which syncs directly with your address book, also makes it easy to share photos and your location.

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Source:

http://blogs.windows.com/windows_phone/b/windowsphone/archive/2013/04/02/big-update-for-viber-on-windows-phone-8-free-calls-with-hd-sound-live-tile-support-and-more.aspx