One of the biggest announcements at WWDC was that FaceTime would be available over cellular data networks in iOS 6. However, it looks as if AT&T customers will not get that feature, at least for free anyway. MacRumors has discovered that when an AT&T iPhone tries to get enable FaceTime over cellular, they get the above error saying that you must contact AT&T first. This is close to the same error that users get when trying to start tethering.
AT&T has issued a statement on this issue and says, “We’re working closely with Apple on the new developer build of iOS 6 and we’ll share more information with our customers as it becomes available.”
This does not appear to be an issue on Verizon iPhones, either.