FaceTime will default to Wi-Fi if you have an internet connection but will supplement with data usage unless you turn off cellular data. FaceTime without Wi-Fi uses cell data, but does not use call minutes. You can turn off cellular data usage for FaceTime (see below), then FaceTime will only rely on Wi-Fi.

Does receiving a FaceTime call use data?

How Much Data Does FaceTime Use? Actually, FaceTime doesn't use all that much data. A FaceTime call uses, at most, about 3MB of data per minute, which adds up to about 180MB of data per hour.

How much data does a 1 hour FaceTime call use?

A typical FaceTime call usually uses about 3MB of your cellular data per minute or 180MB of cellular data per hour.