You can use FaceTime over both Wi-Fi and cellular Internet. If you choose to use FaceTime over your cellular Internet connection, the data you use counts towards your monthly data plan cap. If you use FaceTime on a regular basis and aren't on an unlimited data plan, calling over Wi-Fi could be a more cost effective solution.

The FaceTime Service

The FaceTime service is free to use on your iPhone, iPad, iPod and Mac OS X computer. You need a phone number or Apple ID depending on the device you use. FaceTime cannot be downloaded on third-party devices, and each caller must be using an iOS device.

Data Usage

FaceTime consumes approximately 3MB of data per minute of use, counting both uploaded and downloaded data. An hour of FaceTime usage uses about 180MB of data, whether you're on Wi-Fi or cellular Internet. The difference is that Wi-Fi doesn't count toward your cellular data plan usage.