Having a landline is starting to turning to be constantly harassed by cold callers these days.
So I’m thinking about making incoming calls landing on a voicemail controlled by a custom computer script where my phone would only ring after the audio captcha is successfully solved.

But of course, the problem is they are humans from low‑income countries most of the time, not bots. So since I’m expecting calls only from guys working in ɪᴛ, I thought of something they typically can’t afford : a fast cell phone since the applications they can use on their work computers are restricted.
In that case, I’m thinking about something like :

If this call is important, and in order to prove its aim is not commercial, please dial the last 5 digits of [big random number] factorial followed by the # key.

But then, the problem is a genuine caller might not have the idea to use cuda or OpenCl, in order to solve the problem efficiently on high‑end hardware.

So what kind of better captcha like challenge which can be solved over voicemail I should use ?
Rules : every generated text can be turned to speech and anything that can be programmed is possible, but the foreign low‑income worker from a company illegally spoofing the original phone number needs to be disrupted.

  • Background : I had an Adblock style system in the past. But the government set up harsh laws against call calling so most companies started to illegally spoof their phone number : if you try to call back the caller, you will typically land on the line used by an individual from the state official phone book. Jul 13, 2021 at 11:05
  • What proportion of genuine callers are you willing to lose? Jul 13, 2021 at 14:38
  • The comments under my answers have completely muddied the problem you are trying to solve. Jul 13, 2021 at 18:16
  • @motosubatsu I agree to lose calls about time of the day. I don’t want to lose calls about relative passed away or I definitely want to do business with you. That is, if the matter is really important, they will solve the challenge. Jul 13, 2021 at 19:14
  • @WeatherVane no it’s you who supposed there are other way than using a captcha challenge for the problem. If I say so, it is. Jul 13, 2021 at 19:16

2 Answers 2


Asking a clever question not only risks losing genuine callers, but is insulting, as if to say "If you are too stupid to answer my question I won't talk to you."

I suggest you set up a fake switchboard and give out a PIN to your known contacts as if it is an extension number:

  • My number is 01234567890 extension 1234.

Your script detects the PIN and routes the call to your phone if correct. Otherwise a message states:

  • Please enter my extension number, otherwise enter 6789 to activate voicemail.

which then does silently activate voicemail if that code is entered. In effect, you have a public PIN 6789 and a private one 1234.

This should get rid of most spam callers, allow people to leave messages, or speak to you, with minimal inconvenience or anything off-key.

  • The problem is I’m looking for an apprenticeship. I need to receive calls from people I never talked to before. Don’t need a voice mail for whitelisting phone numbers, as even the pstn allows to block calls by id. A point is most bosses think an apprentice in ɪᴛ still in college is unable to write and configure such voicemail scripts so this is a proof of my habilities more than something unusual. Jul 13, 2021 at 17:58
  • 1
    If you are looking for an apprenticeship, how do you think callers will react to being asked a question by a machine that they have to go away and look up before you condescend to speak to them? It's like you are interviewing them, it's the wrong way round. Jul 13, 2021 at 18:04
  • 1
    No, when you are looking for your job in IT you say "my phone number is that" When they call, the number you have told them will be routed through automatically. Anyone who has not been given the number (and extension), i.e. is dialling at random, won't get past your script. Jul 13, 2021 at 18:11
  • 1
    That's for email addresses. For phone numbers they just make random calls by machine, and when one gets answered it is patched to a human operator (if available). Jul 13, 2021 at 19:31
  • 1
    Anyway good luck with the job hunt. I am sure the world is going to beat a path to your door. Jul 13, 2021 at 19:37

I have a voicemail machine on the landline which I leave active, and I can see caller ID too (unless blocked). If I recognise it as a usual caller, I pick up. Otherwise voicemail kicks in and I can hear them speak, and if I want to talk to them I pick up and cancel the voicemail, replying as if I only just caught the call. If they have already hung up, I can call back if I want to.

So, you could keep it simple and record a message to say, for example:

  • Good day, please state your name and your company name as this call is filtered electronically, before being routed to me.

The 'filter' being you, and you pick up if you like what you hear.

Hack #2. Suppose you are in UK and the harassment is coming from another phone area. Ask for the first 4 digits of the phone number to be dialled in again for your script. If it is an international call, they won't be the same as from a domestic call. If they rumble that, they might not realise that UK domestic phone numbers begin with a 0 which is omitted when dialling from outside the area.

  • But in my case, I need to not call back many times. And with the number of useless calls I m receiving, I would still fill bothered too many times. And again, those companies are spoofing phone numbers of mainland users. Jul 13, 2021 at 13:34
  • In that case, try my hack #2, asking for the first few digits, which you also program into your filter script. Jul 13, 2021 at 13:53
  • In France, all numbers have the same length. Those foreign companies takes care to call you using a spotofed number of the same region or even city. Jul 13, 2021 at 14:08
  • I meant, the first few digit of your number, not theirs. If it's an international call and they include the international dialing code, it won't be acceptable to the script. UK numbers are all the same length too except when called internationally, which they might not realise. The spam callers I get are so stupid they call me multiple times even when they got a definite "no." Quite often too, the spoof ID hasn't enough digits to be a real phone number, so you could check that too. Jul 13, 2021 at 14:15
  • But in my case, I need to not call back many times. Don't call them back then. How is that an objection? If you turn the the voicemail volume down low, you'll soon learn to ignore it and your ear will pick out only what you need to pay attention to. Jul 13, 2021 at 14:50

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