Second this. When it gets a call that’s on your blacklist, it will play a recording of the tones used to indicate “disconnected number” or “no longer in service.” The tones can cause auto-dialers to remove your number from their list.
Usually, a telemarketer will call once and I will never hear from them again.
Works very well, but you need a landline and a computer.
ETA: I reread the OP and see that he/she is using a cell phone, so PhoneTray won’t work for that.
Some telemarketers use pretty sophisticated equipment, able to detect (not perfectly) if their call is answered by a machine (usually a long message) or human (short message, like “Hello?”) or no message at all. The software can act accordingly, either hanging up, playing a message back, or routing the connection to a live agent. Presumably it can also enter the number into an internal database for future reference (call again, don’t call again, possible fax machine, etc.), although I doubt if any do that.
Then there are the robodialers that are programmed to call the same number list repeatedly, every few hours or days, without being concerned with the response or presence in any DNC list, national or internal. My theory is their cost is so low that even if the reward is a few dollars out of a million calls, it’s still profitable and they are immune from prosecution.
Some of these outfits provide “services” to multiple vendors, as evidenced by the varying sales pitches that emanate from the same sources. It’s all about volume. Extreme volume. Extreme annoyance. Money for nothin’, and the chicks for free.
I have a CPR CallBlocker sitting right next to my work phone, since I work at home.
It’s a piece of hardware that goes inline with your phone with a CID display and a big red button that you whack when you want to blacklist a caller. The device hangs up the phone and then adds the number to the blacklist. Future calls result in no rings in the attached phone and one ring on any other extensions.
It makes hanging up much easier–in the past I thought it was better to say “put me on your do not call list” figuring that any other response would cause them to put my (live) number back in the queue for tomorrow. These days I don’t need to interact with them: If I hit the button, I will never hear from that number again.
I feel a bit of satisfaction when I hit the button (one guy on Amazon said he put a skull-and-crossbones sticker on his), and the one-time ring in the house is actually pleasing in a “gotcha!” sort of way.
I do try to help others, so when I get a true scammer or junk call I check 800notes.com and log the number there for others to see (usually just another “me too” message, but it helps).
YMMV, of course.
That concept is of limited use. I have a list of several hundred phone numbers of telemarketers that have called me. None have been used more than once.
No unlike the list of several hundred drug URLS, all hawking the same products, each one used only once. Blocking each does nothing to prevent them trying again.
Hmm, but leaving your computer on all the time is problematic as it could be hacked that way.
How do you figure? It’s behind a NAT firewall. Not impossible to penetrate, but I am not a high value target, so I doubt it justifies devoting a lot of resources to hack.
Posting to the SDMB means your are connected to the internet. You could be hacked that way as well.
Sure, and I am sure you take proper precautions. But for every one person that does, there’s a dozen or a hundred that doesn’t and their machines get taken over as zombies.
Which is why the general public* should be told to turn their computer off when not in use for more than a hour.
aka “idiots”. 
Reducing the number of hours it’s on doesn’t eliminate any threat, just reduces the chances. If your computer is that vulnerable, don’t you dare turn it on, ever. What if, just at that moment, someone hacks into it?
Either you don’t understand the technology, or you are just paranoid.
And you can call me an idiot if it makes you feel good. Doesn’t change anything.
I use and app called Mr. Number. I’ve tried other call blocking apps, but I like this one because I can set it to block (or let through) all numbers from a specific area code, rather than having to enter long lists of whole numbers. Also, I can set it to send certain calls to voicemail and others to just hang up on (or let through). I’ve currently got mine set to block everyone except my contacts and numbers from the local area codes. I get zero telemarketing calls.
Apparently our mileage does vary.
I just clicked through all 120 numbers currently in my blocker (which shows a repeat-call count in the upper right corner) and manually counted 93 blocked callbacks. The most was 21, for “Private Number”, with some 877 number coming up as the next most popular at 7.
Most of them had one or two callbacks.
It’s an imperfect system, but it keeps me happy. I smile each time I hear the sound of a call being blocked.
So, let’s say you use your home computer 2 hours a day. Thus, 90% of the time you are not using it. Thus that reduced your chances by 90%.
But even so, even if your computer has been hacked, turning it off means it wont do more damage as a zombie slave.
Are you under the impression that a lot of end user computers get hacked just by virtue of being connected to the internet? If someone is not opening emails, clinking on links or downloading programs, their likelihood of being hacked is nearly zero.