Some auto-attendants (like the one in my PBX) have two digit extensions I can specify. I can simply say, “We do not accept any sales, political, charity, poll or market research calls. Telemarketers please hang up now. To speak with Bill Smith please dial extension 72”.
In my experience, human telemarketers will not ring through when hearing such a message. The robots are 100% blocked.
Not good enough? Even with really sophisticated voice recognition, how many robo-callers could handle this? To speak with Bill Smith please dial the extension number 12 multiplied by 6”.
Or this? "To speak with Bill Smith please dial the extension number that comes just before 73."