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One of my favorite communication apps right now is Viber, a free download from the App Store that allows you to make voice calls over Wi-Fi or 3G to anyone in the world who also has Viber installed.
What makes the app so convenient is the way it integrates with your existing contacts. There is no need to create a username, build a friend list, or share login details. Once you install the app, Viber automatically identifies which of your contacts are already using the service. From there, you can choose to place a Viber call or a regular carrier-based voice call, all from the same contact screen.
It is simple, seamless, and incredibly useful for international communication. If you have not tried it yet, I recommend giving it a look.