How to keep default to "Prefer Wi-Fi and mobile data" on iOS phone? It keeps changing to "Use carrier only"!

The "Make and receive calls" setting keeps resetting to "Use carrier only". I set it to "Prefer Wi-Fi and mobile data", then a week or two later it will reset to "Use carrier only". I'm outside the US, on iOS phone, and using a foreign SIM, so I obviously don't want to use carrier. No option to set iPhone to no number, so I have to set it to my US number, even though it isn't connected to it at this time. It worked fine before when my iPhone didn't need to be associated with my linked number. Any troubleshooting tips to keep the "Make and receive calls" defaults from switching?