"However, now the service is already available to select users, most of who have an unlimited data plan with the company and uses the 4G LTE networks when doing so."
Umm. That is the exact opposite of my situation. Tom's I think you are incorrect here. They said the reason I CAN'T use facetime over 4G is because I have an unlimited plan. They tried to get me on a shared or tiered plan in order to use it. Of course I told them where they can put their tiered plans.
BTW you can just jailbreak your phone and get a tweak called 3G unrestrictor (works for 4g too) and it will trick your phone into thinking it's on wifi when it's actually on cellular data. Voila!