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Can BT trace withheld numbers?


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Old 07-06-2016, 00:09
Gerry1
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Tackling nuisance calls can be problematic because all the so-called watchdogs and regulators are utterly useless. Many officials are simply gamekeepers waiting for the opportunity to take a generous settlement and turn poacher with one of the organisations they supervised. Some like the ICO are just under-funded, some like Ofcom have a relationship with those whom they are supposed to regulate that's just far too cosy.

The telcos could easily block calls with spoofed CLIs, e.g. 00000, 088888888 or with invalid numbers e.g. those that have too few digits or are in invalid number ranges. They can obviously detect spoofed CLIs because 1471 doesn't go on to invite you to return the call by pressing 3. So why do they still deliver what are obviously nuisance calls?

It's because they get paid to deliver them, so the telcos have no interest whatsoever in tackling the problem. Even worse, they positively welcome nuisance calls because they can then 'blackmail' the victims into renting expensive network services (Caller Display, Anonymous Call Rejection, Choose to Refuse) and/or buying expensive hardware (call blocking telephones) just to avoid being bombarded by continual nuisance and fraudulent calls. These interruptions are bad enough when they just distract you from what you were doing, but for shift workers (e.g. nurses) and those with loved ones that are vulnerable or ill it must be a nightmare.

However, there is a very useful feature that can do the trick. It's Automatic Call Trace but it's seldom made available by default. You'll have to persevere to get it, because most call centre staff will say they've never heard of it ! It works because there's really no such thing a withheld number: the number is transmitted all the way to your local exchange but with a flag set 'don't revel the calling number to the called party'. (In fact, two numbers are transmitted, the 'real' one and a 'presentation number' to which calls can be returned. For example, BT call centres use an 0800 presentation number which returns a message to the effect 'BT called but you were out, don't worry, we'll call again if necessary'.)

After receiving a nuisance or threatening call from a withheld number, you dial 1477 and the offending number is stored at your exchange for subsequent investigation / prosecution if relevant.
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Old 07-06-2016, 09:03
chrisjr
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However, there is a very useful feature that can do the trick. It's Automatic Call Trace but it's seldom made available by default. You'll have to persevere to get it, because most call centre staff will say they've never heard of it ! It works because there's really no such thing a withheld number: the number is transmitted all the way to your local exchange but with a flag set 'don't revel the calling number to the called party'. (In fact, two numbers are transmitted, the 'real' one and a 'presentation number' to which calls can be returned. For example, BT call centres use an 0800 presentation number which returns a message to the effect 'BT called but you were out, don't worry, we'll call again if necessary'.)
If you have the right sort of digital connection to the exchange and a switchboard that can suck out and display the data burst that accompanies every call you can see these two numbers.

I don't actually think there is a flag saying "show" or "don't show" the CLI that gets passed on to the recipient. From what I have seen the CLI field is totally blank if the caller withholds the number. The other number that is used for billing is of course always there.

We once did a few test calls out of idle curiosity to see what the difference was between dialling with or without the 141 prefix. Without it the callers number was displayed in the CLI field. With it there was no number displayed at all.
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Old 07-06-2016, 12:03
Gerry1
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I don't actually think there is a flag saying "show" or "don't show" the CLI that gets passed on to the recipient.
Agreed, the CLI is always transmitted along with the call. Using 141 to withhold a number is only an instruction to the terminating exchange not to transmit the CLI to the called party.

The key point is that most people believe that withholding a number means that nothing is transmitted and malicious calls cannot be traced, but they are mistaken.
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