Every phone number I use is now on a telemarketing list of some sort. No huge task there — it's easy enough to electronically generate every possible phone number in a given region, feed it into an automated dialer, and start plugging away. I just did it in a spreadsheet and it took all of two minutes. That's an old trick and I would guess a surprisingly effective one. The legalities may be questionable, but since we're talking about telemarketers here, I'm going to assume skirting the edges of legality is already the order of the day.
I have three phone numbers tied to my workplace and two tied to me personally. One each from these two categories are cell phones. For those, I've set up the most frequently abusing numbers as contacts with a custom ring tone of absolute silence. So basically they can call all they want and the only way I'll even know is by looking at the missed calls list. And, I don't have voicemail set up on either of the cell phones, so I don't have to waste time retrieving their spiels about car insurance, warranty extension, lowering credit card and mortgage rates, and so on.
My criterion for being a "frequently abusing number" is this: You get only one chance. If you turn out to be a robot or a telemarketer, you get blacklisted.
For the home landline, we have Caller ID service with anonymous call rejection. Yes, the number is on the Do Not Call list, but despite the good intentions behind that idea it's turned out to be fairly useless. We also have one of those cordless phone systems with bases scattered throughout the house. These phones (Panasonic) interpret the name sent with the Caller ID and announce it aloud between rings. So not only can we know who's calling before we answer the phone, we can get this knowledge without even getting up to look. If it's something unrecognized, it goes to the answering machine. Let your machine talk to my machine. And every once in awhile one of us will simply purge the answering machine of its collected garbage.
Once upon a time the outgoing message on our answering machine started with the three tones indicating the number was disconnected. Some telemarketing equipment interprets this as "this number is disconnected, so let's completely remove it from the list and not waste time calling it further." It really did work — the number of junk calls dropped quite a bit while we used this technique. Granted, this was several years ago, so it may not be as effective now as it once was.
As for my work landlines, I'll normally just answer — firstly, because that's part of my job, secondly, because they're my work numbers and I really don't care what lists they're on, and thirdly, because it gives me the opportunity to mess with the occasional call agent who happens to get connected to me. And I'll always choose to speak to a live agent if given the option (if not, I hang up). I'm neither rude nor abusive toward them. I'll let them do their blah blah blah thing and I'll just sit there not saying a word. This normally confuses them, so they'll do their blah blah blah thing again — part of their job, you see. Eventually they hang up, sometimes uttering the mystical phrase "operator disconnected" before doing so.
Call agents aren't supposed to be idle. I'm making them idle by doing this, thus introducing a slight inefficiency into the system. I don't know what that accomplishes, exactly, but I like to think at least some of these places are skin-of-the-teeth operations and my actions are having just enough effect to cause some of them to fold. At the very least I'm throwing their stats off a bit, maybe around a tenth of a percent or so.
I've got nothing personal against the call agents themselves. They're not sitting there calling numbers themselves or doing research on who would be a good person to call. It's all computerized and automated — it's a machine whose fingers do the walking (and the dialing), and the people who do the work have shitty jobs in an industry with a high turnover rate. My beef is against the companies who continue to call — and, I'll add, send fake Caller ID info — despite there being a Do Not Call list, despite there being laws against calling cell phones, and despite every effort we make to prevent it.
Just quit, ok? No one is really interested in what you're peddling, and if they were, they'd call you. The paltry amounts of money you've made have come from people you've essentially tricked somehow, and last time I checked, that wasn't called business.
It was called exploitation.