I ran a quick experiment to simulate traveling and wanting to use my Verizon line only over wifi, never over cellular radio. With previous single-SIM iPhones, this would only work as long as the SIM remained in the phone...after you swap the physical VZW SIM for a local operator's SIM, you can't swap the SIM back and enable wifi calling on the VZW line while in airplane mode, in part because the settings app would not let you get into cellular options while in airplane mode.
But, with the XS and 12.1.1, I can turn on airplane mode and still get into cellular settings, and there I can turn off or on the VZW line, remaining in airplane mode. Turning the line on in airplane mode while connected to WiFi, it will connect in VZW WiFi mode.
So the recipe seems to be this, to avoid any VZW international usage charges:
* Trying voice mail while in airplane mode/WiFi, I get "Visual Voicemail is currently unavailable". I can call voicemail (*86) and get the messages the old way.
- Enable WiFi calling on Verizon line before departing USA
- Enable airplane mode before arrival outside USA
- Turn off the VZW line in cellular options
- Disable airplane mode; install nanoSIM for local provider, use as desired
- When wanting to check something on the VZW line:
- Enable airplane mode
- Connect to WiFi
- Enable VZW line in cellular options
- send SMS messages, make VZW WiFi voice calls, etc. *
- Disable VZW line in cellular options
- Disable airplane mode (return to using local provider)
It seems I have to connect over the air for the VZW line to get visual voicemail to re-enable.
I don’t have it handy now, but there is a Verizon FAQ page somewhere that states visual voicemail isn’t available over Wi-Fi.