If it goes through as an actual Facetime call, then you aren't charged for it.
That said, it can easily fail to go through as a Facetime call and instead connect as a cellular call (either over the cell network or over wifi via Wifi Calling[1] if you have that enabled). If you then add Facetime video to the call, I'm not really sure if the cellular audio call stays active while the FT video goes through data, or if the whole thing transitions to a data call over FT.
That said, the ATT Forums are loaded with posts of this happening to people. Easiest way to prevent it is to just go into your account settings and disable outbound international calls. I'm not sure if they have the equivalent of [2] option in their wireless account portal (the link is for their landline service), but if not you can have customer service do it manually. That way you explicitly prevent your phone from establishing a true international call via AT&T's network (whether via cell service or Wifi Calling).
That said, it can easily fail to go through as a Facetime call and instead connect as a cellular call (either over the cell network or over wifi via Wifi Calling[1] if you have that enabled). If you then add Facetime video to the call, I'm not really sure if the cellular audio call stays active while the FT video goes through data, or if the whole thing transitions to a data call over FT.
That said, the ATT Forums are loaded with posts of this happening to people. Easiest way to prevent it is to just go into your account settings and disable outbound international calls. I'm not sure if they have the equivalent of [2] option in their wireless account portal (the link is for their landline service), but if not you can have customer service do it manually. That way you explicitly prevent your phone from establishing a true international call via AT&T's network (whether via cell service or Wifi Calling).
[1] https://www.att.com/support/article/wireless/KM1063258
[2] https://www.att.com/support/article/u-verse-voice/KM1010583