The service is based on Rebtel to call from local numbers in the country you are in. If you are in Thailand and want to call Sweden, Rebtel creates a local Thai number to call to call, then it connects to the right number in Sweden.
On top of the Rebtel service has also added Internet-based calls, if you have WLAN at the hotel, with Rebtel app free calls to others with Rebtel app on his phone and also make cheap calls to anyone else, even those who do not use Rebtel.
Rebtel app has previously only been available for Android and iOS. But more recently launched a Windows Phone version of the app.
Rebtel is now available for download in the Marketplace.