For me the best defense is to not answer any unknown numbers. They use spoofing to come from your zipcode or one near by. When you answer the phone, then the robo-dialer knows its a real number and will start sending the actual spam recordings. The first one you get is usually just static or beeping tones to keep the call connected. If you stop for a while it should decrease, I still get quite a few though, but it's much worse for a few weeks if I accidentally answer one.