My friends all know my phone # and can call me, so this doesn't help there.
Door-to-door types won't do this; they will ring the bell. Even if they DID do this, why would I want them to?
So who does this leave?
Food delivery types? - again, they will just push the button.
Parcel delivery types? - they don't even push the button half the time; they just drop the package and run (that even happened when the delivery guy was dropping off the cartridge for my dying mother's morphine drip - no signature, drop the package full of a Schedule 1 narcotic, knock twice, and run.)
Law enforcement? Either they will just push the button or they will bash down the door.
Who, exactly, is this for?