Does using facetime use data over wifi?

Hi I have an iPhone 5 and my facetime is restricted to wifi use only and addition to that my cellular data is off and I'm connected to wifi. But after my facetime call there's always MB after it?
Example 21 MB. Even though I'm on wifi and cellular data is off. Can anyone tell me if Im being charged for this? Is it using cellular data? Thank you

If you are connected over WIFI, then you're not using anything off your cellular data, so you will not be charged as part of your cellular data plan.