Santander card fraud alert.

Yesterday I got a call from Santander on the house phone.
A card fraud alert, an automated voice gave me a three digit ID number and then a free phone number, asked me to hang up, then ring them back.
We had only just moved a good bit of cash about so thought hmm ok, but it was odd because they have my mobile number for contact, usually a text.
I Googled the given free phone and it comes up Santander card fraud section.
Went on-line to my account and found a contact number different from the first.
Used my mobile to ring.
Lady checked and no red flag on my card but said that the free phone number given me was correct for Santander card fraud section.
It would appear that when you hang up to ring them back, the scammers do not hang up and you are still connected.
It is an old scam but just be aware.
They rang back four times, trying to get at Mrs.B&B's hard earned dosh.
That is my job, I will thank you very much!
File under "I did not know that":

Analog exchanges (certainly (*1) Strowger exchanges in the UK, probably others elsewhere) did not permit the called party to clear the line (hang up). My understanding of the original reason for this is that the calling party was paying the bill, and there was no effective signalling (pre digital exchanges) up the line (from called party to calling party) to terminate the billing. Remember on long distance calls these calls were patched through manually. This could be used (and relied on) by those receiving calls, e.g. to hang up and pick up the phone elsewhere. I believe this to be the case universally in original analog exchanges (i.e. in every country). This feature is called Called Subscriber Held (CSH).

When digital exchanges came in in the UK, some people had come to rely on being able to hang up and pick up the phone elsewhere, and BT (well, the GPO as it was then) maintained this feature. All modern exchanges have a configuration knob or two ("Called Party Clear" and "Called Party Clear Timeout") which determines whether, and after how long, the called party can clear the call. BT have in recent times set this knob to 3 minutes. This knob has been a feature of System X, System Y (aka Eriksson AXE10) and 21CN phone exchanges. BT use a much shorter timeout on POTS lines configured for analog PABXs. The called party can clear automatically on ISDN2 and ISDN30, and also on mobile and VoIP.

This is described in full in BT SIN 351 ("Technical Characteristics Of The Single Analogue Line Interface") under section 7.1.2:
7.1.2. [Call Clearing] By The Called Terminal​
When a call is ended by the called terminal, the BT network interface will detect an off-line condition (see section 3.1 Off-line d.c. Condition) and initiate a time-out process lasting between two seconds and three minutes. After the time-out period has expired, network initiated clearing (see section 7.2 Network Initiated Clearing) is provided to the calling terminal.​
Calls that are made to certain services (e.g. Number translation services and Premium rate services) are subject to first party clearing. In these circumstances, when the called terminal ends the call there is no time-out process and the calling terminal is provided with network initiated clearing (see section 7.2 Network Initiated Clearing) immediately.​
In other countries, the same knobs are available. The removal of this 'feature' (or introduction of such a safeguard) would have been up to the operator. Eire has a similar telephone network to the UK, as do various current or former British dependencies (another answerer says this is/was the case in Canada), so I would guess their incumbent operators may share the same configuration. However, various cable operators in the UK do not share this configuration. Historically, I believe this configuration has been used in the US (as this patent) would suggest, and was present on Strowger exchanges (see here (PDF) and here).

In March 2014 BT announced it was drastically reducing the time for Called Party Clear. You can find the announcement here (and the text quoted above is post this announcement):

Here are some extracts:
There are potential problems and the risk of fraud when the called party replaces their handset to end a call but the calling party does not. Currently, in this situation, the network will wait between 2 and 3 minutes before initiating call clearing. During this time, the calling party is still connected to the called party. If the called party picks up their handset within the timeout period, they will still be connected to the calling party. Such a feature has always been available on analogue lines to allow the called party to hang up and subsequently re-answer the call for instance when moving from one extension to another. However, this feature has of late been exploited by fraudsters who hold the line open.​
It is planned to roll out the proposed changes using a phased approach across the BT network, starting with the AXE10 exchanges which equate to around one third of the local exchanges currently in service or approximately 6 million exchange lines. It is intended to commence the rollout early April to change the AXE10 configuration for call clearing to 10 seconds with a target completion date of 10 April 2014. Further information regarding the timeline for implementing the same changes to System X and UXD5 exchanges will be made available in due course.​
Note AXE10 is for this purpose a synonym to System Y. Also note AXE10 exchanges are deployed throughout Europe; the setting of this feature is (as previously indicated) a matter for the service provider.

So, the answers to the questions are:
How does this work?​
The caller waits for the called party to hang up, plays a dummy dial tone that disappears when DTMF is received, hopes the called party does not hang up for more than the relevant timeout, and after a few digits have been dialled plays a ringing tone. I imagine Asterisk is eminently suitable for this task.

Why can't the telephone network fix this?​
They can, and in the case of BT are fixing it. Other operators may not need to fix it as they may not offer this 'back compatible' feature.

Does the scammer require specialized equipment or does this work from any landline phone?​
You do not need specialised equipment. However, you do need to dial someone who is on a landline configured for it, and hope they hang up for less than the relevant delay. May people may hang up for less than 10 seconds (which is the new standard it would appear).

Note this technique is not only much loved by fraudsters, but (when the delay was longer) was often used by journalists who, having contacted someone for a 'scoop', would leave the phone off the hook to prevent their competitors phoning the same person whilst they raced around to do an interview.

*1 = it has been suggested that even Strowger exchanges supported this, evidence of which I would be interested in. My research (see here) suggests that in the original Strowger configuration, it merely lit an alarm lamp. Even if this is correct, this is not to detract from the fact that at least some earlier exchanges did not disconnect for a considerable time after the called party hang up.

(Now tidied up, sorry for Giant Wall Of Text).
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Yesterday I got a call from Santander on the house phone.

Had one of those ourselves recently. Somehow, they were able to spoof the caller id so that our phone displayed the freephone number on the back of the (RBS) card. Me being me, I didn't follow their script and argued the toss over their data protection questions until they became abusive and hung up. A quick call to the RBS fraud line confirmed that it wasn't actually the bank (I have had genuine callers hang up on me before, but they tend not to get abusive first).
I got a landline call last night from 'Visa Verification'. A young English sounding chap called Darren, very reassuring - "he was going to ask a series of yes/no questions, no personal details would be taken".
He went on to ask if I'd spent £300 in M&S today, (no) or £720 on Amazon for kids clothes (no again). He then told me that they could not proceed any further as it looked like I'd been scammed and would I please phone the number on the back of the card. I told him I'd do that.
Not being a total prick, I was going to do it on my mobile, so I went to pick it up and was explaining what was occurring to my wife when the landline went again. It was Darren; 'We've just picked up another transaction, £2500 at Direct Line, for motor insurance." I thanked him and told him I was about to call.
Using the mobile I contacted the card issuer (ironically a very Indian voice!) and found that there were no transactions on the card apart from those authorized. Quelle surprise!
I now realize that the second call was due to the time lapse between me hanging up on Darren and making the hoped for landline (which he was obviously holding open) call to the card issuer due to talking to my wife; the intent being to ramp up the panic,
An old scam, but still viable, the f****** English b******.;)