Is this still an issue? I was just about to switch to Cricket but won't if they don't support their apps on current versions of Android. I've got a OnePlus 6 running Pie and don't want to go back to the dark ages before VVM.
Guys, just go into App info for Cricket Visual Voicemail > Force Stop it > Go to Storage for this app > Clear Data.
Next time you run it, it will ask you to set it up > click Allow access and allow all permissions. This fixed it for me and downloaded the voicemails I already had on their servers.