Three words for you all — special information tones. You know the doo-dah-dee sequence before “the number you have reached, EIGHT-SIX-SEVEN-FIVE-THREE-OH-NINE has been disconnected?”
Record this on your voicemail, faintly. Many telemarketers’ robosoftware will think it’s a non-working number and flag the shit out of it. Actual humans will hear your voice over the tone set and still leave a message.