There is essentially ZERO T-mobile coverage for calls or data at my HOUSE!… I can’t believe that in todays cellular world, on the peninsula of the Bay Area, in a very nice town I can find a total dead spot where there is useless coverage.
Local topography can cause that sort of stuff - at my house in the suburbs of a major city I have good AT&T and TMO coverage, but Verizon is useless. Apparently the VZW tower is on the other side of a small ridge and thus no signal here even though there's good VZW signal nearby.
As for your situation, see if the issue is localized to your house or more widespread. I find the TMO Wifi calling to be quite solid, so that could perhaps help you.
Best of luck.