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Comment by cbg0 | Hacker News Reader

I'm not sure I follow. If you've ended the call, and you call the bank number on your card, how exactly would the call get routed to the scammers?

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The United Kingdom phone system has what is called "far-end supervision" where the circuit-switched landline system will only disconnect the call from the receiving caller if the phone where the call originated hangs up.

This trick only works if the receiving caller is on a landline. It will not work on mobile phones.

Sounds like the perfect attack to target against the elderly, who presumably might be more likely to have landlines

This used to work in the US too. Not sure if it still does.

Only some phone systems in the western part of the US had far-end supervision, so far as I am aware. (This is why movies and TV shows from in and around Hollywood show conversations where the caller hangs up and the callee hears a dial-tone. The phone systems in most of California had only far-end supervision. Tom Scott has a good video on this[0].)

Most of the US uses either near-end, where the recipient hanging up will end the call, or both-end supervision, on POTS/landline systems.

0 - https://www.youtube.com/watch?v=bUIiUXvnkUQ - This video was filmed at the excellent Museum of Telecommunications in Seattle, located in a CenturyLink switching office. When travel is available again, I encourage all phone geeks to come here and check it out.

Sounds wonderfully abusive!

So, say, if I called you and you picked up, I would be able to prevent you from calling anyone else by not hanging up indefinitely?

Just great. Will the system not disconnect if you dial 999, too?

It should disconnect eventually. And the timeframe for "eventually" has been changed in recent years.

Originally there was a grace period because of pulse dialling. Each "pulse" is actually a hangup - so the system had to tolerate that hangup != disconnect. But the grace period was far too long, and eventually end-users adopted it as a feature - if you wanted to take this on your bedroom phone instead of your hallway phone, you could hang up the phone, go up stairs, and pick up the bedroom phone.

So now we have two problems. One is that the bug has been adopted as a feature. The other is that precisely because of 999/e911 systems, the phone system is incredibly backwards compatible. Most exchanges still support pulse-dialling - it's never dropped intentionally (some exchanges don't, because they're too modernized. But it's not a conscious "lets turn this off now" thing.)

There has been a move in recent years to reduce the grace period, precisely because of this abuse. But until it's dropped short enough to be a non-issue, my advice for anyone who thinks a call is suspect, is to call the talking clock (123 in the UK). It is a paid service, but I don't like bothering the operator for such things. But if you call 123, and reach your bank, you know summat's up.

> my advice for anyone who thinks a call is suspect, is to call the talking clock (123 in the UK). [...] But if you call 123, and reach your bank, you know summat's up.

No no no no no.

Hang up and use another phone. End of. Any advice that you call another number first or whatnot is bad advice. If such advice got widespread, what would scammers do?

Obviously, they would have a DTMF decoder on the other end and they would patch the call through to the number you called. These are sophisticated people who send fake security officers to people's houses to "pick up the compromised card". Call forwarding is trivial.

Just get rid of the land line. When I move house, I'm not going to have one ever again.

Wow, this sounds awful.

Good that the UK is moving away from this "feature".

(I still remember that to take a call on another phone, you could just leave the receiver up on the phone you too the call on, provided you're not too lazy to hang it up later).

So someone could block you from calling an emergency number (like 911 in the US for example?)

Theoretically yes. It was quite annoying back in the days when people used to pocket dial you.

From what I remember, repeatedly pressing the button under the receiver in quick succession eventually disconnects the call.

I wonder if that had something to do with the calling party pays system they use(used?)

Not sure how it works, I haven't tested it but assume in the UK at least on landlines that both parties must hang up to break the connection.

https://forums.theregister.co.uk/forum/1/2009/09/19/phone_di...

It only works in the UK where the phone call only ends after both sides hang up. The idea is you can hang up go to a different room and resume the conversation. The results are this fraud is possible.

I believe no other country has that feature.

> I believe no other country has that feature.

I'm sure there are other European landline phone systems which behave the same. I don't know about the U.S.

Definitely not the case for Romania. Even though I haven't used a landline in over a decade, if either party would hang up, the call would end.

We used to have this in Norway when I was a kid; I still remember when I found out by accident - it felt like I had been let in on some big secret.

In Sweden both sides had to hang up, not sure how it is now. My mother used it for kids prank calling. She just left it open until the parents came home and wanted to call, then she explained that their kid had been prank calling us.

Definitely used to be the case in Canada. The caller had to hang up: if the receiver hung up it took a (something like 20 second) timeout before the call would terminate. We did used to use that to move to another extension in our house.

Note to kids: we used to have our phones anchored to the wall with these coiled ropes so you couldn't walk away with them To counter that, we had multiple phones in various rooms of the house. They also made the phones so big the wouldn't fit in your pocket as another way to prevent stealing them. They didn't have screens because the vacuum tubes drew too much current and they would get too hot when pressed to your ear.

Here's a report of what looks like a recent case of a scam using this 'feature' in Canada:

https://www.cbc.ca/news/canada/ottawa/line-in-trapping-techn...

> I believe no other country has that feature.

Some locations in Brazil worked like that, depending on which timeframe we are talking about. Pretty annoying.

I guess this depends on the equipment used, not the country.

Norway had it, maybe still, have not touched a landline for decades.

They play the sound of someone hanging up, then play the sound of a dial tone, and you dial again, not having hung up yourself.

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