[joshuatalb]

It sounds like you have Wi-Fi Assist turned on, which does exactly what you describe. To quote what it says in my settings tab - "Automatically use mobile data when Wi-Fi connectivity is poor".

To turn it off, on your iPhone, go to:

Settings > Mobile Data > Scroll to the very bottom > Toggle off Wi-Fi Assist.

[joshstrange]

Yes but data is data, it doesn't matter if I use 3G/4G/5G to talk to a server down the street or across the world, I've never heard of (consumer) bandwidth billing like this.

[ada1981]

Yes this should just be a domestic data charge, of which I am an unlimited plan holder.

I made the call via FaceTime.