Once upon a time you could put the phone company's tone that indicated an out of service number on the front end of your answering machine and it would defeat telemarketers and their dialing computers by removing you number from their list.
Once in a while I would ask them what time they were going to eat dinner and ask for their number so I could call them back, usually ended the call with a click. What has always ripped me about telemarketing is that unlike junk mail, I'm paying for the phone service and they are using my capital for their (attempted) gain. I see that as stealing.
