I was asking for a general case (for more information) but in the specific case that got me thinking about this, all the users in the traveling family have iPhones.
I think they can turn on the Wifi Calling and be set. I just want to confirm texting is free as well. But it should be. (As long as it is to US numbers.)
Of course, as @scottalanmiller reminded me, it doesn't help in the case of an emergency (or if there isn't Wifi), but it's certainly a pretty cool feature to finally come to AT&T.