My prior experience with T-Mobile was that after defining a 911 address, it was still required that you complete a WiFi call while in the US for international roaming WiFi calling to be activated. This seems to also be a stipulation for that service with US Mobile. I asked the reps at USM if they could override the system in my case, but they said they cannot. I wonder if it is part of the 911 network details, but don’t really know. I have told USM I would be testing this over their services on my current trip and they seemed eager to learn the results from an actual user. I switched to a plan that has this capacity only a few hours before departing the States, and forgot about the system confirming my registration. I suspect the 911 system has some very rigid service requirements, but don’t know for sure. I”ll be traveling again in late March and will have an opportunity to wring this out at that time.

edited to add: I searched the Verizon website and found this under 911 references. copy/paste.

• “You must accept the Wi-Fi Calling Terms and Conditions when they are displayed.
• You must confirm, update or enter the US address when it displays (this is where you want emergency personnel to go if you call 911).

Note: iOS smartphones must be connected to the Verizon network in the US. You can't turn on Wi-Fi Calling outside of the US.”