I've had my Nexus 6P since July and have had this problem since the day I bought it. Finally fixed it today by trying a number of solutions. Not sure which one was the permanent fix. Searching the Internet this problem goes back years. Hello? Android? Are you aware of this years-old problem? I'm of the type that prefers bug fixes to new features, especially if the bugs are annoying and in-your-face 24/7. Hello? Android? Are you listening?
This is indeed an ages-old problem, if anyone remembers early Android versions, and even java devices... there was a bit of a conflict between who is responsible for this notification, which Ultimately ended up being deferred to the carrier to properly notify the device to give up the notification icon and alert status. However taking away that option to block seems to have left history to repeating itself.
What I found the cause of this is simply the voicemail system whether it sends the signal diligently enough or not, it is not being received or processed by the Android device. The problem actually with the battle early on was that; to disable the alert on the device would then ignore any subsequent alerts from the carrier. (i.e. any new voicemails would not be alerted, until restart/roam event...)
For someone who doesn't want to break their software to get this thing to go away the easiest and universal solution is to first of all not make it happen: in essence, don't call your voicemail unless you have a good clear solid signal and not a roaming signal, and if that's not possible when you're back in signal range or a good roaming signal that passes the voicemail notification to the device, then; call voicemail, send yourself a message from within the voicemail system, listen to the message, and all other new messages. Disconnecting with a solid signal should solve the problem.
thank you