Any success making Wifi Calls on T-mobile in USA ? (not just enabling option in menu) - Xiaomi Redmi Note 5 / 5 Plus Questions & Answers

Hello,
I have read and found success making options for wifi calling appear in the stock menu when it was absent before, however the phone is nonetheless unable to make wifi calls after enabling. Tried placing phone in airplane and turning wifi on.... or just choosing the "wifi only" option for making calls in the network menu - Resulting message always states to connect to a wireless network to proceed with the call, even though I have an excellent connection and internet works fine.
I have done an exhaustive search for methods to successfully use T-Mobile's wifi calling feature on my phone.
My initial research revealed that the option should work so long as your phone supports wifi calling as well as your carrier.
The wifi calling option was absent (as well as VoLTE) on the stock ROM, so I had high hopes that a custom ROM which included the feature would allow me to utilize the service with T-Mobile (my current GSM carrier).
After a slew of unsuccessful attempts to make calls using wifi vs the mobile network, I finally called T-Mobile's tech support who was actually quite helpful, but essentially stated that wifi calling through T-Mobile requires the carriers own proprietary software to work... in other words, even though T-Mobile likes to promote the ease with which you can BYOD (bring your own device), it would appear that the wifi calling feature is only possible when buying a phone directly from T-Mobile unless a custom rom is built which is at least partially based upon T-Mobile's own proprietary software with regard to wifi calling.
If ANYONE has had any luck at all placing wifi calls on the T-Mobile network domestically in the U.S. using the global/MEG7 version of Redmi 5 Plus, your assistance would be immensely appreciated. I have found that, although my phone does not support multiple LTE bands used by T-Mobile, I am still able to make LTE calls quite frequently in my region using the single supported band.... however, my home is a "dead zone" so to speak and, even though I am smack dab in the middle of an excellent coverage zone, my service inside the house is poor enough that others' attempts to call are unsuccessful, leaving me unaware of the call unless a message is left.
At this point, I am getting ready to throw in the towel so any advice or success stories would be greatly appreciated - or, on the flipside, a validation that wifi calling is just not possible at this time so I don't have to drive myself crazy with a continued search for which a solution does not yet (or may never) exist.
Many thanks for your help!