T-Mobile might not be an official carrier for the iPhone in the US, but that doesn't mean you can't make T-Mobile phone calls on your iPhone.
The company has released its Bobsled app for iOS. Bobsled is T-Mobile's web-based phone and messaging service. With it users can call anyone in their Facebook contacts for free in the US, Canada, or Puerto Rico. Even if you are outside of the US, you can call any of your Facebook contacts in those countries for free.
The service isn't just for iPhone to iPhone calls either. It allows you to call any mobile phone or landline from any iPhone, iPad, iPod touch, or Android device. Right now the service is free to everyone, but as CNET points out T-Mobile does eventually plan to charge for certain calls and/or run in-app advertising. However, right now the company is more concerned about encouraging adoption of the app.
Bobsled is a free download, and it requires iOS 4.2 or later and a Facebook account.