It’s independent of Europe; it’s a function of iOS on your iPhone. If your wi-if signal is strong enough, it will support Wi-Fi calling, and this will be indicated to you with the words “Wi-Fi” next to your normal wifi reception indicator.

There are no additional rates, nothing to do with AT&T. It’s an Apple thing. I’ve had it on forever, use it frequently. Some of the hotel wifi sucks, and the strength of the signal does not support Wi-Fi calling.

When Wi-Fi calling works, it’s great. It allows me to call landlines back in the states like a regular phone. Then the wifi drops, then I start back over.

No reason not to turn that feature of your phone on IMO.