You can make international calls and receive them on Viber as long as you and the person your calling both have Viber. I believe international calls are free on Viber because it is VOIP through the Viber
Let's compare Skype and Viber.
Both are P2P, Viber is beginning while skype is a mature product. Skype signaling and codec are both proven as super scalable. VIber supports push message while skype
Here is a collection of the best Skype alternatives for Linux.
Download the free Brosix Instant Messenger for personal use on Windows, Mac, Linux, Android, iPhone / iPad and web.