8. Call Them Back
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Let's say that you have a VoIP (voice over Internet protocol) system set up: Google Voice, Skype, etc. It has a useful feature that you may not be aware of: the ability to direct calls from specific numbers, any number, to any other number.
Even better, there is literally no way for the telemarketer in question to know who, precisely, they're calling. It could be anyone.
People have been abusing this feature for all it's worth. One guy, when faced with a fax spammer and a surly company that said it was too busy to stop annoying him, redirected the fax calls to the rude receptionist he dealt with. Another redirected the calls from one telemarketer to another telemarketer. Another refers all his telemarketing calls to the Church of Scientology.
Far be it from us to suggest you redirect any calls you get to a sex line. Or anywhere for that matter, considering that doing so is probably a violation of The Telecom Act. And it would be wrong. Funny, and cause all sorts of problems, and probably rack up an enormous bill for the company...but wrong.