Since the list is available to spammers, I nicknamed it the "please call list, my time is important." Nothing at the government level works. Court is out of the questions. The hurdles are way too high. Reporting abuses is useless. But, salvation is nigh.In the US we have the Federal No Call List, which is totally useless. Completely ignored and rarely prosecuted.
My plan is to put those tones on my answering machine AND to have a box that answers and plays them when I see a spoofed number show up.I didn't see a response in my quick scans of the replies as to what those tones are used for (originally).
Back in the "early days" of automated calling, those tones were sent to indicate a not in service phone number, and the automated calling device would immediately hang up and call the next number. It was designed to save time for the automated calling machines. Back in the 80's or 90's I believe, someone even marketed a "tele-zapper" which played those tones on every first ring on your phone, to get the robo-calling machines to hang up. Also, when the calling machines hear those tones, they (usually) automatically drop that number from their lists for future calls.
Besides the tones, there usually is a voice message after, to let "humans" know the number is not in service, assuming humans do not understand that 3 tone message.
I don't know if this concept is still used by automated calling systems, nor if those scammers using VOIP phone systems even respond to those tones any longer. It would be interesting to find out.
Maybe someone could put those tones at the start of their answering machine message and see what happens? Mind you, it may kill automated messages from places you may want to hear from, like automated appointment reminders

The .7z file extension is for a program called 7zip which is free open source and I use for the compression of files and works like winzip which isn't free to use.This should be what you are looking for. I tried setting them up in Audigy and they sound right. Note the duration is 330 mSec for all three tones and the frequencies are 950, 1400 and 1800 Hz. Will try and attach the .wav file. It won't let me.
The uploaded file does not have an allowed extension. The following extensions are allowed: .7z, .asc, .asm, .stl, .asy, .c, .cir, .csv, .djvu, .doc, .docx, .epub, .fzz, .gif, .hex, .inc, .jpe, .jpeg, .jpg, .lbr, .lib, .ods, .odt, .pdf, .png, .py, .rar, .sch, .sub,
Ron
The only problem I see with your idea is you may stop getting calls from everyone including friends and relatives unless you call everyone and tell them what you are doing so they know to stay on the line and leave a message on your answering machine for you to call them back. I get 30-50 of those calls everyday. I thought about leaving my old fax machine on to answer those calls. If those computers thought they were getting a fax machine they would hang up but that fell apart when I started using up all my paper and ink getting useless advertising faxes so that didn't work. The problem with the do not call registry is that those scammers don't care.they are sporting someone else's local number and these are usually foreign in origin where they can't be prosecuted anyway.. No easy way to stop them but they usually don't bother to leave a message so leaving b the answer machine all the time is the best I can do so far. Thinking I might try your idea once I can't take all that phone ringing anymore.In the US, when I dial an inactive phone number, get a loud three tone sequence followed by an announcement that "this number is not in service". The tones do not seem to match the touch-tone frequencies.
I want to try using those tones to discourage the constant barrage of scam callers. Maybe if their computer gets that message they will take me off the list. At least is seems like it is worth trying.
So does anybody know what those tone frequencies are? If I create a circuit to generate that sequence I will share it with everybody.