If you aren’t familiar with Viber, it is a cross platform, free voice call and messaging app (currently available for iOS and Android devices) that you can use to call or text any of your Viber friend for free, on any network, as much as you want. All you need is a 3G or Wi-Fi Internet connection.
I have been using Viber on my Android phone for few months now and from my personal experience I would say it is the best app for free international calls (if you have a free Wi-Fi connection). However, the Windows Phone version, in beta, only have messaging option (no voice call). Viber for Windows Phone (beta) is a free, ad free app that you can download and install from Windows Phone marketplace here. Note: Remember, currently, it is a beta product and limited to messaging capabilities only and there may be some bugs present on it.
