So, I'm reading this thread, thinking
"WTF??? It was announced LAST YEAR that FaceTime became unrestricted for all. No?" So, after I thought about it for a few minutes and conversed with a coworker, we decided that what we actually remembered was this (MR article that pointed to
rumors that it would work with all plans/devices):
https://www.macrumors.com/2012/11/21/att-seems-to-be-extending-facetime-over-cellular-to-all-customers/
Within hours of that story being posted (a couple days before Thanksgiving), both of us tried it. (We both have iPhone 5 w/ legacy grandfathered Unlimited.)
It worked!
I also used FT over cellular during the holidays (T'giving & Christmas).
It worked!
Finally, to make sure I'm not totally losing my mind, I just tried it again.
(Guess what :: It works.)
So, obviously, YMMV. But I was under the impression that the restrictions for most device/plan combos were lifted long ago. My suggestion: try it. Now.