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Comment by jedberg | Hacker News Reader

Just a reminder to never give private info to someone who calls you, even if they seem to have a lot of your private data already to "prove they are legit".

Always call back on a number you look up, not one that they give you.

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Also, don't call from the same phone you received the call on, if on a landline. One time (I can't find the reference) scammers called from the bank, suggested the person called back to the number on their credit card. The person hung up, picked up, and the scammers had held the line, played a fake dial tone, and had someone else "pick up".

In USA telephones, unless you timetravel to "party lines" (when sets of local numbers had the same line, so picking up while a call was in use allowed people to listen or join in), hanging up any one end of a line disconnects the call the departing user from the call.

If the described scam happened, in should have required a simultaneous fault in the phone system. Or more likley, the scammer played a recorded sound of a disconnect+dialtone, which could tricker the target into dialing.

The time required for a good hangup might vary a little bit from exchange to exchange. I recall occasionally being able to transfer to different handsets hanging up one before picking up the other. But not to the extent reported in some anecdotes where one end can hold the call open indefinitely.

This is definitely true. I remember being able to quickly press and release the hangup button on a single phone and if I was quick enough the other person would remain on the line. I don't recall exactly where the threshold was, but I believe it was around a half a second or so.

I remember being able to hang up the phone in one room, run to the next room, and pick up the phone and continue the conversation. My friends and I did this on several occasions. This was in the Atlanta area, in the late 1980s.

Rapidly pressing and releasing the hang up button simulates pulse (as opposed to tone) dialing used by rotary phones.

This is incorrect at least on Bell Atlantic's (and then Verizon's) network in the late 90s. Since there is no double-billing on landlines in the US, the person initiating the call is the only one that can immediately terminate a call to a landline. There's a timeout for the reverse direction, but it at least used to be fairly long.

Someone pulled a trick where they took advantage of this. Had a friend call and keep the line open. Then claim that you have the entire phone book memorized. To prove it, ask someone to name a random name, punch in 7 digits and hand it off to the person who named it. They ask for the name and your friend says "yes that's me" (or "they're not home now if the gender mismatches).

> There's a timeout for the reverse direction, but it at least used to be fairly long.

This brings up one of those cultural things: ever noticed how in movies and TV shows from the 80s and 90s, if the caller hung up, the person called immediately got a dial tone?

It's a trope that prop wranglers, set designers, and writers picked up because the telephone company around Los Angeles (Pacific Bell) had switches that would reset the line state for the destionation back to "ready for call", which meant dial tone, when the origin side disconnected. If the destination side disconnected, the origin would only be disconnected after approximately 20 seconds.

Almost all other exchanges would put the destination--after the origin disconnects--into an off-hook-but-not-ready and then, after 10 or so seconds, play the "if you'd like to make a call, please hang up and try again" recording, then Special Information Tones, then a rapid busy.

Yet because the service in and around LA is what a lot of people in the TV and movie business experienced, it is what got baked into those productions.

> rapid busy

I was a rather violent sleeper when I was young and would occasionally knock the phone off the hook while sleeping. Then I woke up to the fairly loud rapid busy sound. Hadn't thought about that a while.

Interesting. I always assumed that the immediate dial tone after origin disconnected in movies & TV was for dramatic effect to let watchers know that the person hung up the phone.

Now that you mention it, I did vaguely used wonder why some phones took longer to hang up than others. Some, I would hear the receiver go onto its rest, and 'immediately' hear a dial tone. Some, it took a few seconds.

Related to what some other commenters pointed out…

- The delay did seem to get longer when call-waiting became avaliable in an area.

- Sometimes, right after pressing your own hook and then releasing it, I could not dial; I had to wait a couple seconds.

- I never used a system where you could hang up and have time to run to another extension, but I may have known a couple people who claimed they could? If so, I probably dismissed it as "weird".

- My direct experiences were with various regions of just three Bells, so another commenter's remarks about LA/PacBell were interesting.

Thanks, everybody, for jogging my memory a bit.

IIRC, the originating party's on-hook will immediately disconnect the call, while if the receiving party goes on-hook, there is a short but significant delay before disconnect is finalized.

This may have something to do with service offerings such as call-waiting and 3-way, which depend on detecting a "flash" signal.

What? Where do phones work like that? Isn't it enough for one party to hang up for the call to be over?

They used to operate this way in the UK - the line would stay occupied until the call initiator hung up. We used to play with this when I was a kid, but I've not had a landline since early 2000s, so I've no idea if this survived the transition to digital exchanges. TBH I doubt it, and I know lots of people complained about it, because it was really annoying if someone who'd called you hadn't hung up properly as then you couldn't make any further calls yourself.

I believe that potential exploit only work(s|ed) in the UK telephone network, and maybe those of countries developed in parallel using similar technology. Either way, it is a zero-cost precaution so you might as well do it just in case.

Who answers phone calls, let alone from unknown numbers, these days?

I do. I have to. I get lots of important calls from numbers that I don't know. I have a call screener but the scammers play along with that.

I'd say anyone who is involved in anything outside of work probably has to answer phone calls.

It's not very practical for a lot of people to decide that they just won't be available by phone.

I keep my phone on silent 24/7 except for the very rare occasions when I'm expecting a call I don't want to miss.

Sometimes I notice the screen when someone calls, otherwise I call back when I next notice the phone, usually within an hour. If they're busy then, I just send a message instead.

I'm "involved" in plenty outside of work, with an active social life, including regularly meeting new people, volunteering, and more.

I can't remember the last time I got a legitimate phone call except from work. It's been several years at the very least.

I have a lot of medical appointments these days and it's a nightmare how many offices insist on communicating over the phone, calling from a different number than the original one I found. All phone calls must be considered personal attacks until proven otherwise.

My new insurance company cajoled me into "opting in" to their SMS spam for a $100 gift card, but evidently I didn't even need to consent to voice spam.

Thankfully, their CID is "Unknown/Unknown" and my spamblock sends it direct to voicemail.

I do. My mom is terminally ill with cancer and most all of the caregivers, physical therapy, palliative care, pharmacy, oncologist, etc still use good old telephone calls to communicate. Sometimes it comes from a predictable number I can put in my contacts list, but not always. So I turned off the call blocking on my phone so I don't miss important calls.

This has nothing to do with that

Everyone is vulnerable to what this article is about

The reason it is relevant is because after the scammer gets your details, they call you and say they are they bank and need to verify some information, and then you trust them because they seem to have details that only the bank should have.

Then you confirm the scammer got good info.

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