I've been on the Do Not Call Registry since it was founded and have made sure to re-register periodically. My judgment is that it no longer works as well as it once did.
It worked for a while. At first, the drop in the number of telemarketer calls was impressive. And I suppose the registry is useful to the government for mining purposes, and that is a motive to maintain it, even if it doesn't work perfectly for its stated purpose.
I work at home and now get, typically, half a dozen unwanted calls a day on my landline. (Cell phone abuse has not yet started in earnest.) If I'm angry enough I report these calls on the Registry's site, but that takes me away from my work for longer. If I've been tricked into answering, sometimes I tell the callers, "Take me off your list; it is against the law for you to be calling me." They normally hang up before I've even finished that sentence. But the calls continue. Letters about this to my elected representatives have not been effective.
The registry no longer works well — I suppose it does work; I'm still getting fewer calls than I did before the law went into effect. But it isn't working as well as it once did. Some calls are from energy providers, who seem to be allowed to trawl for business under the law. But others are not. There are companies wanting to help me fix my Windows computer — something I do not possess — and fake charities. Some organizations (and some individual voices I can recognize) call repeatedly. Before elections, the calls increase — not only political survey companies, but people who want me to buy something or give them money.
My unlisted phone number has, after years not being findable on the web, is now posted there by someone.
I have caller ID, but many telemarketers use spoofed ID numbers. And there are calls I want to receive — from friends with blocked numbers or from relatives overseas — that don't show any ID or that show a misleading one, so called ID is not a whole solution to the problem.
However, having a landline is still a security, and I intend to maintain it as long as I can. As with other forms of spam — the email, the web-based, and the interpersonal — one uses a combination of prophylactics and poisons to get the job done. No one medicine prevents all disease.
I've been cleaning up an old laptop that still has a phone jack in it, hoping to get it to run a sufficiently current version of Python for me to set the computer up as a phone-call interceptor.