Digital Touch Tone vs. Touch Tone

I am dealing with this issue right now at my medical clinic and the local hospital. Patients are calling to complain in droves because they have an iphone or android and press options on our IVR and nothing happens. They have to hangup and try the call again. Sometimes as many as 5 or more call backs are needed before their tones are registered. I got a T-Mobile line of service to test and called not only our number, but the local hospital whom we have no technology ties to. I reached out to T-Mobile to complain and after back and forth, they told me it was "an iphone issue" and closed my ticket. So I went and bought a T-Mobile android phone and duplicated the issue. I then got another Android phone and duplicated the issue yet again. I also had a coworker with T-Mobile try and duplicated yet again. They came back claiming the issue was now with my Nord N200 device being "international" and "not supporting VOLTE.". I explained that this is a USA phone, with all the carrier updates, and on the phone screen it clearly shows VOLTE in the corner. They then told me I would have to have our patients call to complain about the issue because they can't create a larger ticket unless people call to complain. Never mind that a large amount of our patients are older, and don't have the patience to reach out to T-Mobile. Plus, they will just be told to "try wifi calling" (which doesn't work), or "Reset their network settings" which will clear out all their wifi saved networks and create even more problems, and "reboot the phone". Its honestly a joke. I have never had this issue with my Verizon phone, and my boss at AT&T with no problems.