I have the Verizon iPhone but I might venture to say that there is no app icon for FaceTime...when you go to one of your contacts to call there should be a FaceTime button there for the options under that contact.
One thing I noticed several apps that were deleated previously suddenly reappeared after iOS 9 update ove the air. I suggest you look at what was reinstalled as that may be the reason for losss of space.