As an immigrant living in Canada and having strong ties with Azerbaijan I am using mobile application to contact to my family there. I have iPhone 4s and it was interesting for me which free call application uses mobile data less. During a working day I have no access to wi-fi so I need to use mobile data for a call. The major players in the market is Skype and Viber right now. So I made small research and found out that Skype uses 4 times more data than Viber. Holy cow!!! I was shocked. This comparison is Skype to Skype and Viber to Viber voice calls only. You can check it yourself at Skype's official and Viber's fun websites. For me both have the same connection quality. Then why to pay more?!
Everybody knows Skype. But Viber, it is a new application and it breathes down Skype's neck right now. They just introduced PC and Mac application for video calls. Hope to get video calls in iPhone app as well. Below is introduction of Viber Desktop.