That number isn't tied directly to the carrier. It's an unread count for the app. If no voicemails are showing "unread" in the phone app, it may be a bug in the phone software. The really odd thing is that with Pie the notification dots became tied to the notification shade rather than something within the app itself such that when a notification was swiped away from the shade the count on the app badge would go down (if only one of multiple was swiped away) or the badge would go away (if the last one for that app was swiped away).