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FaceTime registrations seem to use SMS texts. I think they simply meant that unlike AT&T, they do not "waive" the cost of FaceTime SMS'es, but treats them as regular texts, and you'll be billed for every text sent. This could potentially be every time you make a FaceTime call if your registration had expired.
 
I highly doubt they are charging for the WiFi data used during a FaceTime call. This would be a difficult if not impossible thing for a 3rd party to monitor anyway. I'm guessing it just means for any data used in establishing the FaceTime call, i.e. "in every FaceTime call connection." Something was probably lost in the English translation.
 
I'm guessing it just means for any data used in establishing the FaceTime call

It's clearly refering to activating facetime in the settings. To do this, your phone number has to be sent to Apple's servers, which must use some data. It's basically a one-off fee (unles you choose to turn that setting off at some point, restore your phone or lose your phone).

edt: Apologies, I didn't see this bit:
Mobile data usage will be incurred in every FaceTime call connection.
 
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