AT&T started to ease up a bit, and dropped the restriction that only Mobile Share customers could use the feature. The only thing that AT&T wanted was for customers with a tiered plan to use an LTE enabled device to access FaceTime over Cellular. This allowed the feature to be used on a tiered plan with the third-generation Apple iPad, the Apple iPhone 5, the Apple iPad mini and the fourth iteration of the Apple iPad. But that left out the Apple iPhone 4 and Apple iPhone 4S not to mention the Apple iPad and Apple iPad 2.
Perhaps AT&T thought that those who absolutely love FaceTime over Cellular would update to a newer model. Or maybe AT&T was worried about clogging up the ol' 3G network. Whatever the reason was, it is in the past. Starting in a couple of weeks, AT&T will be sending out updates over the next few months so that all Apple iPhone and Apple iPad users on a tiered plan or Mobile Share plan can use FaceTime over Cellular. AT&T has already started updating its systems.
source: AT&T via TechCrunch