I cannot get visual voicemail to activate on GOS on the Pixel 8 Pro. The carrier is T-Mobile. It works fine when running the stock Pixel 8 Pro android OS. I'm fully aware of the carrier features page and nothing there is helpful for this. The Google Dialer (Phone) app with sandboxed GPS doesn't activate visual voicemail either.
The specific errors are:
Google Phone: Can't update visual voicemail
GOS Phone: Can't Activate Visual Voicemail
T-Mobile Visual Voicemail App: Activating.... (spins foverever)

I've tried reverting to stock OS and visual voicemail indeed works there still and after re-installing GOS, visual voicemail again won't activate.

BTW, visual voicemail works fine with T-Mobile and my Pixel 6 Pro...

    Thanks for the reply. Tried that. It gets stuck on activating forever...

    https://grapheneos.org/usage#carrier-functionality

    "GrapheneOS aims to work with all carriers that are officially supported by Google on the stock operating system on Pixel devices. Wi-Fi Calling, VoLTE, Visual Voicemail, MMS, SMS, Calling and 5G (SA and NSA) all are supported, however some functionality may not be usable due to Google not supporting carriers on the stock OS officially or due to GrapheneOS not shipping proprietary apps required in order for this functionality to work on some carriers."

    This may be the cause of your issue.

    Hi Ernest. Thanks for the reply. T-Mobile's visual voicemail works fine on my Pixel 6 Pro in the GOS phone app. It works fine on the Pixel 8 Pro in the stock Google Pixel Android phone app too. The activation is not working on GOS + T-Mobile + any app or method. If I install the T-Mobile app from GPlay, it runs, but just has an activity spinner that says activating indefinitely. The GOS phone app complains it can't activate, as well as google phone app. I reverted to stock pixel OS and it again works fine: activates and retrieves and play messages, using only the stock google pixel phone app. Didn't even have to install T-Mobile app.
    And again, it works flawlessly on my Pixel 6 Pro.

    I just bought Pixel 8 Pro yesterday and have the same experience. Soon after I moved my SIM from my old phone to my new phone, I got an SMS message from Tmobile saying effectively there's a new version of their visual voice mail and that I needed to download it from: t-mo.co/VMMtmobile. I HAVE NOT DOWNLOADED THIS YET OR TESTED AND CAN'T ATTEST TO THE AUTHENTICITY OF THAT LINK but am posting here in case you want to investigate in your testing efforts. I will be doing this soon. I haven't learned enough about profiles yet to determine if I should install this in the owner profile or if I can instead install it in my daily use profile.

    HTH

      Stupid idea maybe. Try using lucky patcher to make an akp of the dialer app on stock and install it on gos with different name ?

      I just downloaded the latest update to the tmobile app (pixel 8 pro) and it also just spins and says activating.

      8 months later

      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!