It’s been here for over a week and won’t disappear. I don’t even have voicemail enabled 
I have tried restarting my phone as well.
Hi, Notifications can persist due to software bugs, settings needing adjustment, or an ongoing issue with the app or device. Restarting the device or checking app settings may resolve the issue.
Had the same issue. I resolved this by going to settings, the phone app. Turn off badges. Restart the Phone, then go back into settings and turn badges back on
Mine also had the same issue but resolved itself automatically after a while.
Navigate through the complete list when you launch the FaceTime app. Should you possess one. It might have been an unsuccessful FaceTime call.