I recently upgraded to the new $30 plan w/$5 monthly discount. Was having issues with voicemails showing up late and not getting notified that I had a vm before the upgrade. Then the voicemail app stopped opening altogether. This fixed it for me:
Go to Settings - Apps - Voicemail - Storage - Clear Data
Then Open Voice Mail and go through the prompts .
This works for me but only temporarily. As soon as I get another VM, I am right back where I started. No notifications and can only open VM by using *86. Every time I have a missed call, I am now calling into VM to see if there is a message. Very annoying.
I recently got an unlocked S20 and it didn't come with a VM app so I downloaded an app called "youmail" and it's been working well. It forwards your voicemail to their service and then you get a notification that you have a message and you can see and listen to the message within the app.