To my knowledge, the reason telemarketers' (or, for that matter, most centralized call centers) numbers appear on your caller ID as "unavailable" is that they use T-1 rather than traditional phone lines, and I'm under the impression that the T-1 carrier doesn't transmit caller ID information.
Not true. Even VOIP phones get caller ID. It's encoded into the bitstream. If it's off, that means that there's something misconfigured somewhere along the line, or it's deliberately blocked.
At this point in time, I simply assume that every telemarketter who calls me is a scammer or otherwise engaged in shady business practices. I'm on the Do Not Call register, and I'm on a cell phone exclusively, no land line. This means that it is doubly illegal for any telemarketter to call me. This goes for non-profits, "market researchers", and political canvassers as well. The only people allowed to call me are those I give explicit permission to do so. (Doing business with them is considered granting permission for the purpose of the law.)
Most of the time, I'll simply announe curtly that I'm on a cell phone, then hang up. If I'm in a really pissy mood, and extremely bored, then I'll screw around with them; but mostly I can't be bothered.
Although I've never worked as a telemarketter, I've worked some pretty crappy jobs that weren't much better, and I've had friends who worked for them for short periods of time.
One thing to keep in mind is that there are almost no honest cold-call telemarketters. Nearly all of them are running some kind of scam, even if just a low-level one. A huge number of telemarketting call centers nowadays are run from prisons, using prison labour. There was a news story a while back about one prison-based call center being a source of hundreds of thousands of personal financial records leaked for an ID-theft ring operating out of the prison. Aside from that, ex-convicts are one of the largest employee demographics for cold-call telemarketters.
Something else to keep in mind is that a lot of these call centers service multiple businesses who are too small to operate their own dedicated call center. Often these businesses don't really know how these places are run, or the tactics they're using; and many times simply don't care. Likewise the telemarketting call centers are scamming their client businesses as well as the people they're calling.
When I worked at Earthlink, we outsourced a bunch of our tech support and customer service to a call center which had been primarily telemarketting up until that point. Earthlink ended up having to sue them for breach of contract because of the extremely poor way they handled the calls (and this was a company that had a number of its own highly questionable business and employment practices).