It's a self-correcting situation.
Good friends or neighbors call ahead of time nowadays.
It's the busybodies who want the element of surprise.
The only fly-in-the-ointment is if the busybody calls your cell while you're ignoring the doorbell ("Hi Bob it's Floyd, I saw your car in the driveway but nobody is answering the door and I was worried that maybe you're choking or having a heart attack becsuse I said to myself 'now why won't they answer the door when I'm ringing the bell I mean isn't that rude?' ") in your knickers while eating chips and watching "Battlebots." The same principle applies....there is no law requiring you to answer the phone.
We learned that rule that you don’t have to open the door in 1983. We bought our first house as a married couple. “Jerry,” the old widower next door liked to drop by, but he was always drunk as a skunk, always hinting he’d like to come in and yak, and impossible to get rid of. We humored him the first few times, but kept him outside the screen door. After that we learned to hide.
If you have to answer, “I was taking a crap” usually shuts them up. Or ‘something’ equally private. Be creative.
walking around naked helps.