OnePlus Nord N200 5G - no voicemail notification

I am having the exact same problem.  I had a Nokia N73 3G phone.  T-Mobile replaced it with a OnePlus Nord N200 5G and I have not been able to find a way to turn on the notifications for voicemail.

I did some research on this database and found this from a fellow who had the same problem with another OnePlus T-Mobile phone:

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.
https://community.t-mobile.com/android-9/no-voicemail-notifications-39777#:~:text=Tmobile%20tech%20support%20needs%20to,the%20right%20of%20the%20time.

I have not gotten back to T-Mobile Customer Service to provide them with this possible solution.

At least for me turning on visual voicemail solved the problem.  I now get the "tape" icon at the top of the phone display if a voicemail has been left.

I did not want to install Visual Voicemail app, so I followed RedRover’s advice, and it was a successful fix.

HOWEVER, after much personal experimentation, I discovered that for the “tape” icon to appear in the Status Bar at the top of the screen the phone MUST BE CONNECTED TO THE INTERNET, EITHER THRU YOUR DATA PLAN OR THRU WIFI, Y THE PHONE APP HAS TO BE OPEN. Apparently T-Mo Voicemail notifications are NOT carried on the regular T-Mo network signal, although Missed Call notifications are. This is quite inconvenient for those of us who do not have a data plan with our T-Mo service. (I'm on Android 12, BTW)

FYI, I am on a legacy T-Mo Prepaid plan that does not have a data plan as part of my package. This is how I discovered this T-Mo notification anomaly, and it took hours of personal experimentation to figure this out, so I hope my note here saves someone else some time and frustration. If you have a full-time data connection with your service, you probably wouldn't even have noticed this "tape" icon problem, once your voicemail was reset by the T-Mo rep.

BTW, you will first see the little blue phone icon, notifying you of a Missed Call. Once you clear out the Missed Call notification, the icon changes to the tape icon IF the caller left a voicemail AND you are connected to the internet (the Phone app will already be open by you clearing out the Missed Call notification.)