I returned recently from a trip to Europe during which I used several data-only Airalo eSIMs, After returning home and resuming use of my Google Fi service, a new-to-me error message popped up on some (but not all) occasions when I tried to call someone from my contact list. The message said I wasn't registered on a network and could only make emergency calls. Despite that message, I was always able to make my desired calls by keying in the desired telephone number manually. I was also able to web surf as usual.
Yesterday I Googled the error message and tried several of the suggested solutions--rebooting the phone, turning Airplane Mode on and then off again and verifying I was on the latest version of software. The problem persisted. I also verified my Google Fi SIM was set as the priority; it was. As I looked at the SIM Priority setting, it occurred to me my Belgian eSIM might still be active, which it was, and that I should turn it off. When I did that, the oddball error message disappeared.
I don't know why having the Belgian eSIM turned on but set as non-priority caused a problem with my telephoning service, but it seems pretty clear it did. I'll have to remember to turn off any non-expired eSIMs while I'm on the plane flying back from Europe.