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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 recei

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.

__blockcipher__ 6y ago
Sounds like the perfect attack to target against the elderly, who presumably might be more likely to have landlines
StillBored 6y ago
This used to work in the US too. Not sure if it still does.
- techsupporter 6y ago
  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 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.
romwell 6y ago
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?
- soneil 6y ago
  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.
  - Robin_Message 6y ago
    > 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.
    - Graham24 6y ago
      Just get rid of the land line. When I move house, I'm not going to have one ever again.
  - romwell 6y ago
pkaye 6y ago
So someone could block you from calling an emergency number (like 911 in the US for example?)
- robjan 6y ago
  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.
jdofaz 6y ago
I wonder if that had something to do with the calling party pays system they use(used?)

__blockcipher__ 6y ago

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

StillBored 6y ago

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

techsupporter 6y ago
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 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.

romwell 6y ago

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?

soneil 6y ago
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.
- Robin_Message 6y ago
  > 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.
  - Graham24 6y ago
    Just get rid of the land line. When I move house, I'm not going to have one ever again.
- romwell 6y ago

pkaye 6y ago

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

robjan 6y ago
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.

jdofaz 6y ago

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