Use phone's built-in Visual Voicemail on T-Mobile USA?

Is there any way to get Visual Voicemail working using the Phone (dialer) app on T-Mobile in the USA? I have two open-market phones - one a factory unlocked handset from China (Unihertz Titan) running Android 9, the other a formerly-locked (to Telstra, if that matters) Galaxy S10+ running Android 10 - and neither can access visual voicemail through their in-box Phone apps. I've never had this problem before, with older Androids (back around the Android 4 days) or with the Windows phones I used until EoL (everything from WP7.x through W10M, VVM worked out of the box perfectly). T-Mobile has a VVM app on the Play store, but even aside from the inconvenience of needing a separate app for what all my previous phones had integrated, it is... bad. It's rated 2.5 stars, and it earns it (not all voicemails showing up, generally clunky and unreliable). I've been on the phone with T-Mobile, Samsung, and even Telstra; none could help. On the Titan, the VVM screen always says "Visual voicemail is not activated yet, please try again later". On the S10+, the VVM toglle is just outright missing from the built-in phone app. I haven't rooted either phone, and they're both running stock ROMs. I have ADB installed, if that's needed.