One of the applications that I have been enjoying using that just came to the Windows Phone Marketplace today is Viber. Currently voice calls is not yet supported in it since it is in beta form but according to Viber it is coming in a future update.
I have compared it to WhatsApp and so far the experience has been a lot better. It can search your contacts for friends that are currently using Viber so that you can easier chat with them. So far I have replaced WhatsApp on my Lumia with Viber
According to the press release, the current version only allows Viber Messaging aka "allowing users to exchange text and high-quality photo messages and share locations with other Viber users."
Viber is available for Android, iPhone®, BlackBerry® and now for Windows phone®.
We are always introducing new features, platforms and possibilities.
Viber is completely free with no advertising, and we value your privacy: read our policy at http://www.viber.com/privacy
Get it, you’ll like it.
* 100% FREE: Viber and all our features are completely free, with no subscriptions or purchases. Avoid mobile phone “bill shock” – text other users, on any network, as much as you want. All you need is a 3G or Wi-Fi Internet connection, and friends who get Viber too!
* SIMPLE TO USE: Viber just works, and lets you connect with your friends whose numbers you already have. It synchs with your mobile phone number and contact list to instantly show who already has Viber. Never again must you seek out your own contacts, create new usernames, or build new lists.
* VOICE COMING SOON: We promise that as soon as the full version is ready, we will make it available so will be able to use Viber for Windows phone to also make free calls to any other Viber user, anywhere in the world.