Re: It actually MAY block them depending what callback service they're using
"I don't see the "Press 5 or go away" working with the usual cold callers. They are persistent beyond belief even when it is clear you are not interested from the offset."
It works for me. Since I implemented an Asterisk exchange at home with a similar setup three years ago, my home cold calls have gone down to exactly 0. Maybe I'm lucky but I suspect that they give up and move on when their dialler "detects" an answer machine.