Not sure if you found a fix but I had the exact same problem with the oneplus 9 pro.
Visual voicemail is NOT needed. I believe they just say that so some people will eventually sign up for premium services.
Here is what worked… Tmobile tech support needs to completely reset your voicemail. You will lose any saved voicemail messages. Then reboot your phone and now voicemail notification now shows up on the top of the phone display as a little blue phone icon, just to the right of the time. It just took me 2 hours of tech support for them to comprehend the problem and then fix this. Again, visual voicemail is NOT needed.
