WiFi Calling not working suddenly

I have been a T-Mobile customer for many years. Wi-fi calling worked perfectly until a software update from T-Mobile around the middle of 2020 --  and it has not worked reliably since then. This same problem exists on many different makes of phones - Apple, Samsung, etc. I too live in an environment with poor cell coverage and have long depended on wi-fi cell phone connection with T-Mobile. Since that "update" my T-Mobile phone refuses to remain in the "wi-fi preferred" setting and constantly reverts to "cellular preferred" sometimes just minutes after I select "wi-fi preferred".  Sometimes Wi-Fi works, but I never know if it is working or not. Worse yet, OFTEN wi-fi incoming calls go straight to voice mail and my phone never rings. These problems have been brought to T-Mobile's attention over and over. The same issue occurs on various wi-fi networks. For a long time T-Mobile denied there was a problem. And they have tried blaming the phones, other apps, wi-fi networks, etc. In fact I have tried different phones provided by T-Mobile - but ALWAYS have had the exact same issue - meaning this is a software issue. It really needs some high level technical work. It should be very easy for T-Mobile to explore the problem by setting up a panel of many different phones and observing the issue at a test center. The other answer is to stop advertising wi-fi as a reliable available communication method with their cell phones. Such as advertising  "it is sometimes possible to connect on a wi-fi network." But not reliably so. To advertise the service, and have it not work properly for many months, is a dangerous situation for users like me. I use an Android phone but see many on-line complaints from Apple users. To deny that the problem existed, in the past, was irresponsible and dishonest. You can delete cache, and turn anything on and off.. it won't help. Something is deficient in the operating system… a CHANGE from what was working PERFECTLY prior to the abominable update. Every T-Mobile customer who uses or depends upon wi-fi connectivity should be reporting whatever difficulties they are encountering. To me - the wi-fi issue is a much more significant one than 5G availability. Once we start traveling internationally, the unreliability of wi-fi connecting could be a costly one to us, as well.