Since I installed GrapheneOS a month or so ago the Visual Voicemail section in the dialer app never worked. So I've had it turned off. I was on the Mint Mobile reddit page and noticed their pinned post for FAQs included instructions for Visual Voicemail so I thought I'd give it a try. To my surprise the instructions worked! So I thought I'd post it here knowing there are quite a few others who can't get Visual Voicemail to work in the GOS dialer app. Hope this helps.
https://www.reddit.com/r/mintmobile/comments/wm1ynh/mint_mobile_faq_updated/?rdt=55251
Enabling/Issues with Visual Voicemail
On Apple make sure there is not a carrier update - Settings > General > About > Carrier update (if available)
On Android, Disable Visual Voicemail (VVM) in your phone settings
Note for Samsung phones: There is a Phone by Google app and a Samsung Phone app, you have to open the Phone by Google app to change settings, get from store if not installed.
From a Home screen, open the Phone icon (Phone by Google app, icon is blue old-school phone on white background). If this phone icon is not on your home screen, swipe up to display all apps then tap the Phone icon.
Phone App > ... Menu icon > Settings > Voicemail > Visual Voicemail.
Reboot phone
Disable Wi-Fi
Open the phone app, go into Settings, go into the Voicemail menu (make sure VVM is OFF for now)
Tap Advanced Settings, then Setup. Take note of the Voicemail number listed here, it will be important in the next steps
Dial in to mint voice mail listed, make sure your password is set and a greeting is selected (can be default or one you record)
Open the phone app, go into Settings, go into Calling Accounts.
Tap on Mint Mobile, then Call Forwarding. Note, this menu might take a few tries to get into, may get a few errors before it loads up successfully. It also takes LONG to load, so be patient
Under Call Forwarding, you'll see "When busy", "When unanswered", and "When unreachable" listed. All three of these should have the same phone number. Ignore if these numbers were different and Replace all the phone numbers in Call Forwarding with the number you got in Voicemail Advanced Settings
Return to the Voicemail settings, and enable Visual Voicemail.
Wait 5 minutes then have someone call you and leave a new message, see if you get notification and VVM.
Re-enable Wi-Fi
You can also use 3rd party app like T-Mobile Visual Voicemail app, YouMail, or Google Voice.
Also note that not all Androids support Visual Voicemail on Mint, and sometimes an OS update will add Visual voicemail support and other times it removes it. This is why Mint says Visual Voicemail is not supported on all Android devices, as they cannot control what OEMs do. Mint is working with Google engineers for them to make update to get Pixel visual VoiceMail working. If your native phone app does not support, you can use 3rd party app like YouMail or Google Voice.