It took FaceTime a long time to start working over cellular, but now that it has it's made it incredibly convenient to talk with your kids while you're walking down an out-of-town street, or check with your partner before buying something for the home or office. Of course, with cellular access comes cellular data usage, and if you're on a limited plan, you may want to be careful how much you use FaceTime over 3G or LTE. Luckily, the iPhone and data-enabled iPads make it easy to turn FaceTime over cellular on, and off.
To disable cellular data access, just repeat the procedure and toggle the switch to OFF.
If you need more help with using FaceTime on your iPhone, iPod touch, iPad, or Mac, check out the articles below, or head on over to the iMore forums.