I don't know who they are, but the call a LOT. I don't answer calls I don't recognize. If they really wanted to "talk" to me, they'll leave a message, but they never do. They just always hang up.
I turned off my answering machine. I don't want these a**holes to get the impression somebody is answering my number. If I don't recognize the number, I don't answer it. I don't try to return the call for fear they pick up on that somehow.