Virtual Office Numbers can be configured to either Play a Main Greeting or Directly forward your caller to an Extension.  When one of your Virtual Office numbers are configured to Ring to an Extension directly, that Extension’s Delete button is hidden to prevent accidental deletion of the Extension which would break your call routing. If you wish to delete an extension that is currently being used in a Direct Forward call-flow, you must first  change the Virtual Office number to forward to a different extension, or to Play your Main Greeting. Then, you will be able to Delete the Extension.