They know they're going to get some kind of answer when they call, so they'll keep trying for a live voice. They're likely thinking you're away from the phone and you'll answer another time. Most pests need a live voice for anything unless it's a bogus IRS call where fear is their motive for a catch. Those are perhaps the only ones prepared for an automatic answer. If these guys suddenly aren't even getting an answering device, you've got a chance at getting them tired of calling you. Other than finding out your number has an answer, there's nothing to gain leaving messages.