As you are probably aware, Apple’s new FaceTime video calling feature does not work on unofficial carriers (like T-Mobile USA). This is because of the way these carriers handle the SMS sent from/to the Gateway for activation of service. So, if you are on an unlocked iPhone 4 looking to use FaceTime, here’s a way to get it done.
PushFix, a jailbroken iPhone app developer has released an iPhone app named MiTime that will help you out in this regard. The application fixes the inherent issue with recognizing the SMS from the gateway so that FaceTime is activated. However, because this procedure will require the developer to send you two international SMSes, the app itself is not priced cheap and is available on the Cydia store for a price of $5.99.
Do note that this app will only work on authorized devices though users from across the world (except Cambodia) can make use of this app to activate FaceTime on their unlocked iPhone 4. Here’s a video on how the process works.