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Back in the days when people still had faxes, it was not
uncommon to be phoned by a fax machine, which would then
whistle and peep at you. If you whistled back, it was possible
to confuse it, so it would persist for longer before hanging up.
Today, alas, fax machines are far and few between.
all phones these days use dual-tones, both for dialling and for
allowing the user to enter numbers or select menu items. The
dual-tone system uses pairs of audio-frequency tones in
various combinations to represent digits (or other keys). The
tones needn't be generated by the phone itself - any suitable
sound-generator, held close to the phone's microphone, will
work just as well.
Now. It is possible to whistle and hum at the same time, and
it is also possible to whistle one note whilst humming another.
Doing so in a controlled way is extremely difficult, but not
completely impossible. It should therefore be possible to dial
a telephone number using nothing but oneself (and a phone,
obviously, but not its keypad).
[theircompetitor, Feb 18 2019]
DTMF at Wikipedia
A reasonable explanation [neutrinos_shadow, Feb 19 2019]
||There are in fact people who can do this having problem
finding info, but has to do with ability to generate the two
||You didn't need two tones to confuse it for a few seconds, it would try to latch on the carrier tone for a bit. To attempt to confuse a modem at all you did need to be a bit ... baud.
||My dad perfected the "boop" made by supermarket barcode
scanners. Saved/stole him literally pounds worth of stuff
over the years. I'm sure he'd approve.
||//There are in fact people who can do this// That is very
||read the linked article -- I've seen better pages on that but
they may have link-rotted. I was involved in building
telephony systems late 90s early 2000s and I remember
reviewing DTMF related material and seeing mention of
||//whistle and hum at the same time...
Prime bagpipes territory then.
||Umm, didn't phone phreaking occur somewhere between
the Earth coalesced and the Teenage Mutant Turtles?
||Somewhere (I thought it was at Wikipedia (link) but it's
not...) I read about why those particular frequencies are
used. Something to do with no harmonics between any of
them, so all tones and combinations of tones are completely
unique and non-interfering.
Also, sometimes (in a completely random and uncontrolled
way) I can whistle 2 tones at once, and there are people
who can sing 2 notes at once.
||So, a Scotsman piper dials up a Mongolian. The
Mongolian says, hang on, let me conference in my