Why can't I clear the missed call notification icon from my notification bar HTC U11?

[AC Question]

I have tried to clear the call by pulling down the notifications and swiping it away, I've tried clearing it with the "clear" button, I've tried calling the number back and texting it, and I've tried deleting the call from my call history and no matter what I try the annoying missed call icon persists. When I use the clear option, it clears away the call and leaves a white rectangle. When I close it the icon is still on the notification bar. Help!