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Viber for Android is an Android application that lets you make free phone calls and send free text messages to other users that have Viber installed.Free international calls and text messages to other Viber users using 3G or Wi-Fi.Viber integrates seamlessly with your existing address book,when you use Viber, your phone calls to any other Viber user are free, and the sound quality is much better than a regular call. You can call any Viber user, anywhere in the world, for free and now also text them.Once activated does not require a PIN, username or any additional "in application" purchase .
Some features Viber for android :

All Viber features are absolutely free and do not require any additional “in application” purchase. Once you and your friends install the free Viber application, you can use it to talk and also text as much as you want. Just makes sure all your friends get Viber. All you need is an Internet connection: 3G or Wi-Fi where available. On top of that, Viber doesn’t contain any annoying ads.

It doesn’t matter where you or your friends are, be it on the same network or in a totally different country.You can talk for as much as you want, for free. Just make sure your friends have Viber too.

You know your friend’s phone number, so why bother with yet another username and password? Viber uses your phone number as your “identity” and lets you make free Viber phone calls to any of your friends that have Viber using their phone number.

Unlike applications like Skype, Windows Live Messenger or Fring where you need to “add buddies” and have them approve you, Viber behaves just like a phone. Just like you don’t need to “add” someone in order to call them, you don’t need to add your friends in order to call them on Viber. If you know their phone number, then you know their Viber number, and you’re ready to go.

To make sure you always get incoming calls or messages notifications, we strongly recommend you to keep Viber running in the background. This doesn’t drain your battery or use up memory and improves your Viber experience.
-Sound quality

Viber uses state of the art technology in order to make sure the sound quality you get is much better than GSM or a regular phone call.
?Download  Viber for Android