Just for the fun of it, I tried using FaceTime over cellular and low and behold, it's working perfectly.
To be sure, we do NOT have a shared AT&T Data Plan. Ours is still the original unlimited Data Plan we had with our original iPhones (1st gen.).
Family plan for voice, separate unlimited plans for data.
Haven't tried if this is just between phones on the same account or anyone, but it's the one that matters most to me.