Does AT&T Iphone 4S use data on Facetime over wi-fi?

My girlfriend just got an iPad for XMAS and I have a limited data plan and I'm always on Wi-Fi on my iPhone and you can only use Wi-Fi over Wi-Fi. I just started using FaceTime for the first time with her since she's out of town and I just noticed when I check the details on the call it says how much memory each called used. Do you know if this used my network data or is just stating the data that was used over Wi-Fi?

Added (1). I just turned off my usage of any cellular data over Wi-Fi in my settings just to be safe for any future incidences.

It's just Wi-Fi memory used.

In case you have internet caps on your home internet

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