I'm sorry I can't offer you a solution, but maybe my experience will at least give you some idea why you are having voicemail issue...
My provider is Sprint/T-Mobile (previously Sprint). I am having the same issues with new my SG 21A phone.
After many hours of frustration, chat and in-store efforts, I have learned the DEFAULT in-phone voicemail app is NOT COMPATIBLE with the T-MOBILE NETWORK! The T-Mobile employee downloaded the Visual Voicemail app which is - per T-Mobile employee - the CORRECT/COMPATIBLE voicemail app for T-Mobile Network.
When asked, the T-Mobile store employee could not explain WHY ( or HOW!) an INCOMPATIBLE voicemail app is the DEFAULT voicemail app on a T-MOBILE NETWORK! I decided to let it go and just be grateful I now had the correct voicemail app for the T-Mobile network.
Unfortunately, the INCOMPATIBLE DEFAULT voicemail app does NOT allow the COMPATIBLE Visual Voicemail to work - despite setting it up as the default for my phone! It is NOT possible to change any settings in the INCOMPATIBLE voicemail app!!
In the 10 yrs I've been with SPRINT, I NEVER had such frustrating issues! Employees - in-store AND chat - don't seem to have a clue how to fix the problem! NOT impressed T-Mobile! Time to start ooking into other options...
I'm having the same issue: switched from Sprint to T-Mobile SIM and now T-Mobile tells me my S20 FE is "not compatible" with (apparently default) premium versions of Visual Voicemail and Scam Shield apps. Can you provide any details on the Visual Voicemail you got from the T-Mobile employee? I have v18.104.22.168199-MR dated 2021-02-08 11:23:15, with the T-M magenta-colored icon. I recently got upgraded to Android 11 but had the same issue with Android 10. TIA.
Now I have TWO voicemail apps and NEITHER one works!
Why T-Mobile sells a phone with a T-Mobile-incompatible default voicemail app seems to be the big question nobody can answer!