This type of service is known as "call through telecoms" and works in the same way as calling cards and most international phone cards. The advantage of the Rebtel app is that it handles the call through portion automatically. All a user needs to do is make the call using the Rebtel app rather than the standard Phone app. You can either select a contact from the address book (drawn from the People Hub) or direct dial via the on-screen key pad.
The biggest advantage of Rebtel compared to direct VoIP services is that you don't need a data connection - given 3G / WiFi coverage is not universal that's a very useful attribute. It's worth noting that your operator may still charge you for making a local call, although this will be a lot less than an international call. Moreover, many users will have a quota of inclusive minutes as part of their monthly contract, which means there will be no direct charge for the local call.
Rebtel say their international call rates are an average of 98% lower than typical operator rates and are up to 60% lower than Skype Out rates (a Skype premium service which lets you call a land line from your Skype account).
Other key features of the Rebtel app include native address book integration, display of current Rebtel rates alongside contact information (based on dialing code), and support for sending low costs SMS (196 countries supported, with average cost savings of 60% over normal operator rates).
Marketplace description:
Join more than 17 million people around the world and make low-cost international calls with Rebtel for rates up to 98% lower than your regular operator’s. The Rebtel app connects your calls with local numbers and uses your wireless plan minutes instead of your data plan, letting you enjoy talking to your loved ones and not worrying about you being disconnected because of a spotty data connection.
Rebtel can be downloaded from the Windows Phone Marketplace for free.