@u0421793 bro, I'm reading this from reputable sources. Certain phone routes were vulnerable to a single 2600 tone. Phreaking is said to have begun with the discovery of this exact SINGLE 2600Hz tone.

I agree that the most common signalling mechanism was dual tone but that's not as fun since extremely few people can do that with their voice. And yes, this is why blue boxes exist.

@u0421793 @sudorandom
IIRC, the 2600Hz tone was part of the system that allowed you to make long distance calls without operator intervention. Time frame was late '60s, early '70s - when in-band signaling was in use for phone equipment on one end of the wire to tell the equipment on the other end what was happening. (Like sounds made when a coin was dropped into a pay phone.) DTMF was used for long distance network signaling starting around 1959.

The 2600Hz tone was used by the local central office to tell the equipment on the remote end that the call had finished.
The hack was that if you played a 2600 Hz tone into a phone that was making a long distance call, the far end would hang up and the near end would not - you could then use a special DTMF box that generated network tones (not the same pairs that later were used by touch tone phones, but the same idea - network DTMF was decades earlier) to dial a new number on the far end. By timing the disconnect correctly, you could make the remote calls without the local end generating any billing information.

Lots of link rot if you go searching for exact details.

@u0421793 @sudorandom In the US AT&T introduced DTMF in 1963 but the signal in question wasn’t a DTMF signal and wasn’t meant to be generated by a phone.

It did take nearly 20 years for DTMF to become widely used though because AT&T had an incentive to introduce technology slowly until they lost a couple of famous court cases.

@lpbkdotnet @u0421793 @sudorandom there was also a *lot* more official secrecy about the British telephone network and it was harder to get any information about it until well into the late 1990s. There were also filters in some cases to prevent 2280 Hz being sent down the line from the subscribers end, and even alarms that sounded if it was detected. Phone phreaking was considered fraud against the public purse so risked heavy punishment.

By late 90s it was possible to bluebox some Global Majority countries via Country direct (dial 0800 89nnnnn and send 2280/2600 tone down when you heard the pip) but I never did it as it didn't seem ethically right when you could already legitimately get dirt cheap phone calls compared to the 80s/90s...