Thousands of patients with T-Mobile unable to use our IVR (DTMF tones not transmitting, requires multiple calls)

I work for medical clinic with about 600 users. For almost 2 years now, we have been having issues where wireless callers from T-Mobile/Mint/Metro PCS and all of their MVNOs, have the inability to get through our IVR system. They will push options in our main auto attendant IVR and it will not register anything they press. Sometimes it will take multiple calls before finally, they are able to transmit tones. I am able to duplicate this issue on my T-Mobile test line on my iphone (signed up for the 90 day evaluation) as well as a test android phone. Wifi calling disabled, strong LTE or 5G signal. Issue happens all over our City, and some of the surrounding cities. Both phones (iphone 14 Pro Max and T-Mobile Revl and OnePlus N200, all the latest ios and carrier updates). Our support vendor for our phone system and our SIP carrier keep telling us the issue is on T-Mobile's side. I had worked with T-Mobile support for several weeks, and they tried to blame the issue on iPhone, stating that ever since iOS 13, the DTMF Long tones are no longer a thing, and to verify that our system can handle short tones. I don't buy the DTMF thing, because why would Verizon and AT&T iphones not have any issues with our IVR. Plus, I can duplicate the same issue, multiple times, on an Android phone. They have now proceeded to kick the can down the road and told me to have our patients reach directly out to their support. Ok, we have done that, for 8+ months, and it got us nowhere, except 100s of angry clients with T-Mobile. I know at least 50 of our patients have switched carriers because of this issue.

I am literally at my wits end with this issue. Does anyone else out there have a similar experience or issue? If you did and solved it, please tell me what you did so we can try it. We have tried the whole Network reset, Wifi calling on and off, etc. I have made about 200 test calls in the past 72 hours, on both my T-Mobile phone, and another carrier. The other carrier, all 200 calls transmit DTMF without issue, the T-Mobile phones, its absolutely hit and miss.