No cell calls over WiFi

My parents’ house does not get cell service. They have Xfinity wifi.

Cell calls do not go in or come out 95% of the time. But not 100% which is very confusing. Occasionally when a call is successful it just disconnects randomly after a bit.

It’s not the phone model, this is the third phone I’ve owned where this has been the case. I have phone calls set to go through WiFi correctly in my phone settings.

This same thing happens at another Xfinity house without cell service that I work at so it is definitely them.

I’ve tried all the tips I’ve found online.

I need to be at their house more often now but I cannot be there without a working phone because of my job. Please if anyone has a solution, I am desperate. Thanks