Apple’s FaceTime feature is currently only available over WiFi, this could possibly change in the future, and from the looks of it when Apple’s releases iOS 6 you will be able to make FaceTime calls over 3G.
The guys from 9 to 5 Mac have been testing out iOS 6, they were using US mobile carrier AT&T, and when they tried to make a FaceTime call over 3G they were greeted with a message to contact AT&T.
When they contacted AT&T they were given a statement which said that AT&T were working with Apple on the developer build of iOS 6, and that they would share more information with customers in the future.
So it would appear that AT&T may offer FaceTime over 3G, although the service will have to be activated like their personal hotspot feature and will probably mean that some sort of extra fee will be needed.
The feature was also tested on a Verizon iPad, and they were able to make FaceTime calls over 3G without issue, so it looks like AT&T may be the only ones charging for the service, as soon as we get some more information we will let you guys know.
Source The Verge