They often can't. A call can come in from any number of sources. Not to mention call forwarding and all that stuff. Telcos would love to kill robo dialing, but

They often can't. A call can come in from any number of sources. Not to mention call forwarding and all that stuff. Telcos would love to kill robo dialing, but they usually do it by requiring a certain percentage of calls to complete and last longer than 6 seconds. If you want to hurt them, answer and hangup immediately.