I, like many others, are more than a little annoyed at being interrupted with whatever I'm doing by some telemarketer calling me on the phone. I hang up on them immediately, or give repeat offenders a dose of the sharp end of my tongue so they won't want to call me back (which seems to be about the only way to persuade them to remove you from their lists).
Even if was interested in what they're marketing, I wouldn't want to do anything to encourage them, and there isn't any way to verify whether they are who they claim to be. They've already behaved in an untrustworthy way (see the list of examples below), the rest of their business ethics are probably just as bad.
So now I have a new approach when convincing friends that responding to marketers is a bad idea: Instead of telling them that it's risky, I tell them that nearly all of them are frauds. That they are going to rip you off. I don't just warn them to be wary, I tell them consider them to be outright con-artists, as the default point of view.
If you don't like them either—and I don't know of anybody outside of the marketing game that does—then the next time someone talks about getting calls from a telemarketer to you, tell them that you've heard that they're all thieves and frauds, that they steal money and abuse credit card details, that those claiming to be collecting for charities are lying about it. Let the gossips, those who believe in every conspiracy, put these people out of business for us.
Then, just maybe, we might be able to answer the phone knowing that it's probably somebody that we want to speak to, rather than listening to it ringing and having second thoughts about whether we want to answer it.
Spread the rumour…