Unlimited Cellular Data Plans are too expensive, as compared to unlimited plans offered by ISPs (Internet Service Providers). If you are a heavy facetime user on a contract plan with your cellphone network provider then it is highly important to keep track of data used. There is no rocket science in this, since both the iPhone and iPad allows the user to see how much data they’re using using for FaceTime over the cellular data network. However there is one thing to keep in mind that the stats your iDevice provides isn’t really specific for whether the video call was made over your cellular data network or WiFi. However you will need to refresh statistics for viewing after a particular call so you can see how much data that call used on your cellular data and let you figure out how much it costs you.
Video Consumption and Calls use the most amount of data since they are large packets. There are two basic ways to monitor and see how much data is being used for Face Time calls.
Viewing Face Time call data usage for an individual Face Time call from the Face Time app
Open the Face Time phone app on your iDevice. Then go to the Recents tab of previous calls in the bottom left. Choose the person you called to view the data usage for the audio or video call you want to see. Tap on the Info button (the small “i” on the right of the contacts name). The data usage amount will show under each Face Time audio or video call.
This is one quick way of viewing the data used, but it isn’t practical to count and sum up what and how much has been used all together, some data may be utilized over Wi-Fi and some over the Cellular. Therfore, the best way to do this is to use an app; see here Monitor iPhone and iPad data usage