If you have missed one or more calls, you will see a "missed calls" popup on the top right hand corner of your Polycom phone that flashes between the popup and the date.
The easiest way to remove this popup, is to hit the down arrow on the circular wheel that is below your home button (button with house icon on it). Hitting the down arrow, shows you a listing of your recently missed calls and by "viewing" this list, you will be able to get rid of the popup.