NowhereFast I know it's been a while since the last post but just wanted to share my experience.
I set up a Pixel 8 and a Pixel 8 Pro with T-Mobile prepaid service (in the US) a few weeks ago for a couple of people. Visual Voicemail would not activate from the stock GOS phone app nor the Google phone app.
I downloaded the official T-Mobile Visual Voicemail app from the Play Store, and the T-Mobile VVM app has been working perfectly for a few weeks now on those phones. But it will only work if sandboxed Play Services are installed. If PS are not installed, the VVM app will not activate, and you will be stuck on the activation screen forever.
I definitely wish that VVM would work on the stock GOS phone app (or in the Google Phone App) like it is supposed to work, but it currently doesn't work (at least not for me). Hopefully one day it will work again!