They wait for the answer machine to finish, say nothing and hang up.

Depending on what your greeting is, they can get a name to go along with your number for resale at the same time. That's one way these bozos know your name. Answering machine means "they've got a live one here" and two for the price of one.

Political Call from Steven Harper

Didn't leave voice mail. Called them back and it's a political call from Harper. Must be an election underway... so call blocking to the rescue !

Harper & Co.....the king of robocallers. I've gotten calls from this number at least 3-4 times this past week. Robocalls should be declared harassment, if not telephone terrorism!