How would they know it's an international call unless Apple/FaceTime would leek metadata (like number you're calling) to AT&T? Sure they could see that the IP

How would they know it's an international call unless Apple/FaceTime would leek metadata (like number you're calling) to AT&T? Sure they could see that the IP you are connecting to is in Africa, but if you call a US number on a regular phone and that person happens to be in Africa you will still get charged like it's a call within the US (the one out traveling will get the extra charge). Since how would you know that person is out traveling? So how could AT&T know you're not calling a US number?