According to the developer, the jailbreak app should work in almost every country except for Cambodia, and Bolivia (Tigo).
MiTime costs $5.99, which seems reasonable if you want to enable the FaceTime feature that is currently not working on your unlocked iPhone 4. It also includes the cost of sending two international SMS required to activate the feature.
You should be able to receive International SMS to activate the FaceTime feature. (You can use this website to test whether you receive a SMS).
The video below demonstrates how MiTime activates the FaceTime feature:
As always, let us know how it goes.[via MiTime]