Rebtel is an international VoIP company (think Skype... well kinda) that has released tier mobile app, Rebtel, over on the Windows Phone Marketplace. Rebtel's Windows Phone app joins other mobile solutions that are available for the iPhone and iPad as well as Android phones and tablets.
The Rebtel app features address book integration and offers to provide rates up to 98% lower than average operator calls and up to 60% saving on international text messaging to more than 196 countries. You do need to register to use the services and if you're a Facebook fan, you can sign in through your Facebook account. The Rebtel service even has Paypal support as a billing option.
It should be noted that Rebtel isn't a traditional voice over internet protocol service. The app won't use your Windows Phone data but instead uses your voice minutes to call a local relay that in turn connects you to the destination number.
Rebtel is a free app that you can find here at the Windows Phone Marketplace.