I had the same problem (no notifications of voicemail). I have a Pixel on Verizon.

It turned out to be a conflict of some sort with an app called Calendar Snooze. This app allows you to select a snooze period after a calendar event goes off (great app!!). However, after the latest Android System update (Pie), Calendar Snooze got turned off somehow. The Voicemail problem started at the same time.

I hadn’t noticed that snooze wasn’t working (don’t ask!), and two sessions with a Verizon Tech didn’t solve the Voicemail problem.

Once I realized that Snooze wasn’t working and turned it back on and reset its notification actions, and then reset the Voicemail notification actions, everything worked!