Flagging a bad phone number.

There isn't really a way to flag a bad phone number.  You can add a comment into that lead's activities, but there isn't a way that I have seen to be able to flag that person's bad number on their profile.  This would be a great feature to have because it will cut down on dial time if I know the number is bad and don't have to go looking through notes to see if it's bad or not.