Viber, a popular instant messaging and calling service with more than 175 million users, has updated their Windows Phone 8 app to version 2.2.0 to include support for free calls with HD sound (VoIP). Viber uses your existing phone number as your user ID, making registration painless. It also means Viber can automatically detect which of your existing contacts is already using Viber (by syncing your address book to the service).

Viber uses Windows Phone 8's new VoIP APIs, so that like Microsoft's Skype preview app, voice calling is fully integrated into the platform. In practise what this means Viber can run in the background and that incoming and outgoing calls use the same UI experience as calls made over the cellular network.

So why might you want to use Viber? Well the key point is that calls and messages between Viber members are free. Messages and calls are routed over a data connection (WiFi or 3G), so operator charges may apply if you're using a 3G connection. This means you can potentially save significant amounts of money of per call or per messages charges. The exact saving will depend on your phone contract (e.g. whether you have unused inclusive minutes) and who you are calling (e.g. international number).

Viber isn't the first app to offer free messaging and calls, but, aside from Skype, it is the first on Windows Phone to offer deep integration with the platform. This means the app doesn't not have to be open in the foreground to receive incoming calls, rather incoming calls will alert you in the same way as an incoming cellular call (i.e. showing the standard incoming call screen / notification).

Viber is also notable for supporting "HD sound quality". This is related to the codec used for encoding the audio and means that call quality should be better than standard cellular voice calls (although an increasing number of operators now also support HD voice).

In addition to the calling features Viber allows you to send text, picture, and location messages either to individual or two groups (up to 40 people can participate in one group). This messaging / instant messaging functionality is the same as previous version of the app, although there have been a few tweaks to the user interface. There's also better platform integration with the ability to pin a conversation to the start screen (improved Live Tile support) and the addition of lock screen notification support (number of new messages). 

Windows Phone Store description:

Viber lets everyone in the world connect. Freely. More than 175 million Viber users call, text, and send photos worldwide - for free. Viber is available for Windows Phone, Android, iPhone® and other smartphones. We are constantly introducing new features, platforms and possibilities. Viber is completely free with no advertising and we value your privacy.

Viber integrates with your phone to provide the best VoIP experience. It syncs with your contact list, automatically detecting which of your contacts already have Viber. Your phone number is your user ID, so you don’t need a username and password. 

Viber can be downloaded from the Windows Phone Store for free.