I have an enclosed entryway in the front and an enclosed screened porch in back. Both have locked doors so no one can come into them. Unfortunately, that means that I can't hear when anyone knocks on either the front door or back door. I went to Lowe's and found one of those electronic doorbells (the kind that has the button for outside and a bell that plugs into an electrical outlet inside the home.) It has a ding-dong for the front door and just a ding for the back door. It has been a Godsend. I would find it incredibly rude if someone walked in through a closed door to knock on the inside door, but then, I'm an incredible privacy freak.