I don't know if this gets rid of them but it sure irritates them: my answering machine sounds like I've actually answered the phone-
Hello (pause, pause, pause) leave your message after the beep.
It's too funny because you can hear the callers start talking right after my "hello" and then they get interrupted with the rest of the message. They end up hanging up.
There's a lawn service that calls 2-3 times a day and this went on for weeks. What was so funny is that they got caught in it every time. Finally, one day, the guy that called said "I hate your answering machine".
My family doesn't like my message so when they call I have to say, "Hi - it's me" so they know I'm not the machine.