When you try to turn on Wi-Fi Calling on iPhone (Settings->Cellular->Wi-Fi Calling->Wi-Fi Calling on This iPhone, using iOS 11.0.3) you get an interesting pop-up message:
Enable Wi-Fi Calling?
When you join a network, the country where the network connection is made may be sent to your carrier and your cellular identity may be sent to the Wi-Fi network operator. The city may be sent to the carrier for routing short code calls. This can be changed in Settings.
What does this mean? I was just going to turn it on to help make better calls in my house, though I guess people might use Wi-Fi calling to save on charges when traveling abroad. But why would the carrier and "Wi-Fi network operators" need to know each other's information? And what is this other setting they are referring to?
Finally, another question related to this same page: The second option is "Prefer Wi-Fi While Roaming." What if you want to prefer Wi-Fi even when you aren't roaming?