There is still a caveat to "Facetime Over Cellular," however, one that still attracts a lot of ire. Users who elect to continue on a grandfathered unlimited data plan cannot opt into "Facetime Over Cellular." Instead they must be on a tiered data plan.
Although AT&T has not explictly said it, the limitation against unlimited data users is encouragement for them to move off grandfathered unlimited plans. It is similar to Verizon's recent move: Users cannot buy a device with a subsidy unless they drop their unlimited data plan and move to a Share Everything plan.
When FaceTime over Cellular launched in September 2012, we explained that we wanted to roll it out gradually to ensure the service had minimal impact on the mobile experience for all of our customers.Obviously, users of non-LTE devices will stress a different part of AT&T's network, and there are far more of them, so AT&T's caution is understandable.
As a result of ongoing testing, we’re announcing AT&T will enable FaceTime over Cellular at no extra charge for customers with any tiered data plan using a compatible iOS device.
AT&T said the update should be automatically applied to devices over the next few months, and that eligible users won't need to do anything to begin using FaceTime over cellular.
That sounds like the change will be applied via a carrier settings update. That's a small update, separate from a ROM update, and is very unobtrusive.