I'm also not able to get Visual Voicemail through T-Mobile on my Pixel 7. I initially tried via the built-in Phone app. When it was unable to activate there, I installed Google's Phone app, but it's unable to activate there as well.
Visual Voicemail does work via the T-Mobile Visual Voicemail app, though. But I'd obviously rather not have to use another app.
I was hoping maybe there was some kind of provisioning thing going on and thought that maybe activating Visual Voicemail on T-Mobile's app would make it possible to get it working in the dialer. However, even after activating via the T-Mobile app, it won't activate in the dialer.