Some carriers may disable or restrict FaceTime connections over cellular, to reduce strain on their networks. AT&T, for example, is limiting iPhone FaceTime usage to customers who subscribe to one of its pricey "everything" plans; users with cheaper family plans can't use FaceTime over cellular. On the other side of the spectrum, Verizon places no such conditions on FaceTime. If your iPhone's carrier or plan does allow FaceTime connections over cellular, be careful!
The convenience of connectivity enabled in iOS 6 and other mobile platforms is a great idea, but it typically costs too much to freely take advantage of its promise.
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