I second the opinion that people don't listen.
My home phone number is one digit different than the Child Support Enforcement number. I used to leave a message that said, "You have reached ***-****. This is not Child Support. That number is *#*-****. Please hang up and redial. If you would like to leave a message for HogwartsAlum, please do so after the beep and I will return your call." No one listened. They left messages for their caseworker, left their case number, and one lady even left her SS# on my machine! (I called her back and told her not to do that again and that I erased it.) Every day I would come home to messages that weren't for me.
Finally I had to resort to making a message that said, in a very twangy and goofy voice, "Hiiiiii. I'm a answerin' machine in a private house. This isn't Child Support," blah blah blah. It sounds completely idiotic, but it gets their attention. Instead of messages, now I get some hangups, but it has cut down on the problem significantly.