AT&T is rolling out FaceTime over Cellular for tiered data plans on all devices at no extra cost! Sadly grandfathered unlimited plan users are left out. Previously AT&T only allowed FaceTime over cellular with shared data plans on a LTE device.
Now supported devices such as the iPhone 4S, third generation iPad and newer, and lastly the iPad mini will be available to use FaceTime over cellular once it is rolled out to you. At&t said “the update will be applied automatically over the next few months”.
So for some people, this change is greatly welcomed, but for some reason Skype can be used on all plans for video calls, yet At&t limits FaceTime, which to lots of people and including me it makes no sense.
Source: AT&T Blog