Viber

With Viber, everyone in the world can connect. Freely. More than 400 million Viber users text, make HD-quality phone and video calls, and send photo and video messages worldwide over Wifi or 3G - for free. Viber Out can be used to make calls to non-Viber mobile and landline numbers at low rates. Viber is available for many smartphones and platforms.
Viber is compatible with and optimized for Android tablets! Use Viber on your tablet and phone simultaneously.
On Viber, your phone number is your ID. The app syncs with your mobile contact list, automatically detecting which of your contacts have Viber.http://www.9apps.com/android-apps/Viber/