[OnePlus 6T  Themes, Apps, and Mods] techlogik:
So did more testing on this. What I found to work on the OP6T, including Android Q 10 Beta. I'm on Verizon BTW...this seems to be an issue for us users to get Visual voicemail working. But mine is working fine.

I also use Nova launcher and Tesla Unread, only thing that doesn't work in the Teslaunread showing a missed call/notification number on the icon, it shows in the app, but doesn't display anything except normal alerts/heads up/statusbar.

Installed the following and rebooted:

Magisk Google dialer framework

Magisk Osmosis BusyBox

Magisk systemizer

Downloaded latest Arm 64 in Google drive patches Beta version 41. Surely most will work. From this thread and contributor.

https://xdaforums.com/android/apps-games/app-google-phone-v14-0-175904292-bubble-t3708218

https://drive.google.com/drive/mobile/folders/1BIc1nXEzrrdVCunJYUW50h_peoneMzCd

OP 6T was working normal and VMM already functioning.

1) Installed Google Dialer, opened to set basic permissions but no VVM tab yet.

2) As an extra step. When you first setup VMM, it has to do some customization for Verizon looking at the /data/data app folder and prefs files to make it authorize. I took the VM file prefs from the Android dialer/VVM contents from data folder prefs for the pasted that into the prefs file in Google dialer shared prefs. Who know if this is needed. Didn't hurt it though.

3) Set permissions for "display over other apps" and "system settings modification" for the Google dialer. Make default dialer. Otherwise you won't get floating stuff and VVM.

4) Dialer tested and working except VVM option and tab doesn't show up yet.

5) Last, use a Terminal app to run Systemizer, just type Systemizer and you can list all apps, it will show the Phone app, select and move to priv-app option 2.

6) Rebooted phone, opened phone app and visual voicemail app appeared on the fly as it expanded over to 4 tabs on the bottom and a new voice mail was there. Did a test voicemail and it worked and updated normally/refreshed.

Also, disabled stock phone after force stop. Enabled it again, seems to have no effect on anything as long as it isn't the default.

Hope that helps someone. Key was really the Magisk systemizer app. Had it working prior, but it wouldn't update when a new voicemail arrived and only an old .28 Dark version would do anything with the VVM tab. Now even the latest Beta 41 Google dialer from the above link works. It now updates immediately and shows an unread new VVM also. Again, this has been a big snag for people, especially Verizon customers.

Only other thing that doesn't work besides the Tesla unread, no idea why, it sees the Phone app etc..just won't put a missed number notification....is on the lock screen, when a call arrives, you can't swipe down far enough to decline the call on the lock screen You can swipe up and it works. I think it is the buggy Ambient Display on the Android Q Beta could be it the issue. When the screen is not locked but display is sitting on the notification "lock" screen, it works fine swiping down to decline. Guess what I'm trying to explain, the first Ambient display when locked with Nova Launcher, it won't swipe down and work. But if you are at the next lock screen, it will swipe down and work.