What is a good iphone AND android app for FREE CALLS?

I'd like an app that is both available on the App Store and on Google Play/Android Market that can give free calls to each other.

Added (1). I've tried Skype, and Textfree. It's nice I can call to a cellphone, but, the fact that I have to buy minutes isn't worth it. I still use it for general texting, though.

Skype?

Tango.voice and video calls

Skype, tango, ooVoo.

Viber, both for iPhone/iOS and Android. Lets you make free calls and send free text and photo messages.

For Android: https://play.google.com/...voip&hl=en
For iPhone/iOS: http://itunes.apple.com/...17920?mt=8

Textfree with Voice is simply the best. It gives you ten free minutes to start with and unlimited free texting in Wi-Fi or if you have a data plan. You can download apps that Textfree with Voice advertises to get more minutes.

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