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.
