No voicemail notifications showing up on Phone Icon

My voicemail notification on the app (the little red number that shows up) suddenly stopped working. Cannot get it back to working no matter what I do with notification settings, cellular date roaming, restarting the phone, shutting down the app and restarting, etc. Any other suggestions besides calling ATT? Because they were not helpful. Interestingly, yesterday I was told the my voicemail box was full, although it wasn't. I was able to clear that by following instructions on this website but the absence of notification of voicemails persists.