That is because iOS 6 enables FaceTime calls over a cellular connection, be it 3G or 4G. Interestingly, that is exactly what a recent rumor was predicting. The functionality will be added to the iPhone 4 and 4S, as well as to the iPad 2 and the new iPad. Another twist that has been added to FaceTime is that the user's phone number will be unified with their Apple ID. AS a result, if they get a FaceTime call on their iPhone, they can pick up the call from their iPad or Mac.
It was about time, we must say. Now all that is left is for the new iPhone to come with a better camera.