Unsolicited telephone calls

you hang up, but the scammer does not hang up he stays on the line? upshot is that the scammer can eaves drop on your next outgoing call
no, he can't eavesdrop on the next call. He can however remain connected to your line until it disconnects at the next metering increment, preventing you from getting through to anyone else, and, as you say, may impersonate the next person you dial.

If you leave the handset down for a few minutes, it will disconnect anyway.
 
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They tend to have a recording of a dial tone so when you pick up the phone it sounds like you have a clear line. They stop the recording once they hear you dialling.
 
We have BT Call Guardian. The first time anyone rings they get an automatic message telling them it is in place and to state their name. We have now accepted all family & friends that we want to allow through automatically.
Once a new caller rings they have to state their name and press the hash key. Computerised/Auto Diallers can not do this function so never get through. Callers who do state their name are rejected if we have not contacted them first.
 
I sometimes see phone no's from friends in Australia and Canada, though
That would be when the call arrives in the U.K. on some carrier other than BT, as BT has a policy of regarding overseas caller ID as "unreliable" and won't pass a number on even if presented with it, instead just sending the "International" message. At least that's how BT was for years and the last time I was there; it's possible they've changed policy by now.

Overseas numbers often get corrupted by carriers trying to alter the format to make it "look right" to U.K. users though. For my last few years in the U.K. I was getting daily calls from my now-wife in the U.S., area code 707. When the call came in via somebody other than BT, the number would variously be presented as 1707xxxxxxx (U.S. country code is 1), just 707xxxxxxx, sometimes 001707xxxxxxx (i.e. as would be dialed from the U.K.) and sometimes as 01707xxxxxxx, making it look superficially like a U.K. call from Potters Bar, if one didn't notice that the whole number was one digit too long for that.

Unavailable, no caller id, if I'm not mistaken, is a call from somewhere that uses different extensions through one or two numbers and requires the user to dial 9(eg)to gain an outside line.
No, that has nothing to do with it, except indirectly by virtue of the fact that many of the places who use services which don't pass on the number by design are also likely to be places with a PBX and many internal extensions.

With a reputable company which has no desire to hide its identity, if it's just using a small number of regular telephone lines, the number sent will be either the individual number of each line, or perhaps the company's main number (e.g. if they had 589100, 589101 and 589285 but they only ever advertise 589100 as their contact number, they could have 589100 sent out on all three lines).

Larger companies which have digital trunk circuits can often determine the caller ID which goes out on any given trunk on a per-call basis, subject to restrictions on the number being within their range. This is often tied in with DID (Direct Inward Dialing) facilities where, for example, the main company number might be 589100 but you can reach any person's extension directly from the outside by dialing 589101, 589102, 589103, etc. Outgoing calls will typically either send the specific number related to that extension, or calls from any extension will always send the main 589100 number. Or it can be set to a combination of methods, so that some extensions send their own DID number while others always send the main number.

KenGMac said:
upshot is that the scammer can eaves drop on your next outgoing call
Not quite. If having received the call, you do not hang up for a sufficiently long period of time before picking up the phone to make another call, if the caller has not hung up you will still be connected to him. He can then fake a dial tone, a ringback tone after you've dialed, and then pretend to be somebody else.

In the "old" days there were some switching systems in which this C.S.H. (Called Subscriber Held) condition could be maintained indefinitely if the caller didn't hang up (or at least until an engineer at the exchange noticed the alarm condition and manually released the connection). Other systems would timeout after a set period and release your line anyway, which is how it works throughout the country today.
 
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@PBC_1966
Thanks for the extensive explanations, obviously experienced in telecommunications.

Not sure if you'll know the answer to this.
On one or two occasions I've received a call or text on my mobile and the number shown as the texter or caller is my own number.
How do they do this?
 
Thanks for the extensive explanations, obviously experienced in telecommunications.
Ex-BT years ago, then self-employed doing a lot of telephone systems work for clients. :)

On one or two occasions I've received a call or text on my mobile and the number shown as the texter or caller is my own number.
How do they do this?
Somebody using one of those service providers which do no verification of "ownership" of a number and allows any CLI the person likes to be sent down the line. They probably figure you're more likely to have your curiosity peaked and answer a call purportedly from your own number than from an "Unavailable," some valid but fictitious number, or something silly like 00000.
 
you hang up, but the scammer does not hang up he stays on the line? upshot is that the scammer can eaves drop on your next outgoing call
no, he can't eavesdrop on the next call. He can however remain connected to your line until it disconnects at the next metering increment, preventing you from getting through to anyone else, and, as you say, may impersonate the next person you dial.

If you leave the handset down for a few minutes, it will disconnect anyway.
Are you certain they can't? With the right technical knowledge and software, would imagine virtually anything is possible these days. I think it's prudent to phone someone you know first to make absolutely sure you've cleared the line. That's the advice Ive heard anyway. Paranoia rules..
 
Thanks for the extensive explanations, obviously experienced in telecommunications.
Ex-BT years ago, then self-employed doing a lot of telephone systems work for clients. :)

On one or two occasions I've received a call or text on my mobile and the number shown as the texter or caller is my own number.
How do they do this?
Somebody using one of those service providers which do no verification of "ownership" of a number and allows any CLI the person likes to be sent down the line. They probably figure you're more likely to have your curiosity peaked and answer a call purportedly from your own number than from an "Unavailable," some valid but fictitious number, or something silly like 00000.
I worked at BT many, many years ago when Sir George Jefferson was CEO.

As per previous post I've had calls from 0000 numbers also.
These are usually from overseas(India) companies.
 
Ok, thanks for your indepth answer. I love this forum. Every technical query known to man answered in easy to understand language. :rolleyes:
 
Are you certain they can't? With the right technical knowledge and software, would imagine virtually anything is possible these days.
There's nothing they can do remotely over a normal telephone line to achieve that once their call has been disconnected; it would entail gaining access to the exchange equipment by some other method. It would be possible to bridge you to another of their telephones lines, from which you'd receive dial tone, and be fooled into making your call over that line. Obviously then they'd be able to hear everything, but that's because their original call to you was never released. It would rely on you picking up to make another call within the relatively short time before a forced release of the connection took place (i.e. they couldn't hold you up indefinitely and wait for you to make your next call half an hour later).

It should also be pointed out that the C.S.H. condition described above applies only to the called person on a regular telephone line. The caller cannot hold you up if you're using a mobile phone, an ISDN line, VoIP service, etc. Once you hang up on one of those, the call is dropped immediately, regardless of whether the caller hangs up or not.
 
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