We appreciate you giving it a try, cwipfli,
If you're still not seeing notifications for voicemail, we'd recommend that you enable Airplane Mode then disable Airplane Mode to reset your network connection. You can enable Airplane Mode via the Control Center or in Settings > Airplane Mode.
Also, if you haven't restarted your iPhone recently, we'd also recommend that you go ahead and do so: Restart your iPhone
If you're still experiencing the same behavior you can also Reset Network Settings. Go to Settings > General > Reset > Reset Network Settings. This also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you've used before.
Best regards.