Does each phone number have a unique tone?

[Berkut]

@GaryM

I’m an amateur radio operator (Ham) and when a Ham uses our repeater to make a phone call, the DTMF tones are masked to prevent the tones from being intercepted. Decoding the tones is not too hard.

I think already know the answer to this, but… What would keep someone from listening on the input?

[LSLGuy]

@Bill H.

I heard (but it may be an UL) that some some nefarious commercial once asked kids to hold up the phone to the TV, then played the DTMF tones for some 900 number. In hindsight, that doesn’t sound very believable.

Long ago I used to be in the 900 business, before it was overrun by ripoff artists. The practice you describe is explicitly prohibited by Federal law. Guess how it got to be in the law? Right. Because some unscupulous operators did exactly that.

[LSLGuy]

@jovan

Someone a bit more knowledgeable will probably fill in more details, but it works that way:

The tones you hear are called DTMF signals. Each number does indeed have its own tone, which is made up of two frequencies. If you put all the numbers and symbols on a 3 by 4 grid, each row and column has its own frequency. It looks like this:

Hz      1209   1336   1477 
697      1       2       3
770      4       5       6
852      7       8       9
941      *       0       #

So, yes, someone with a very good ear (or sound analysis software) can figure out the number just by listening.

If you have an old genuine Bell System touchtone phone, you can make it play the individual frequencies.

If you press two keys in one column simultaneously, the circuitry will produce that column tone without producing either row tone.

Likewise, if you press two keys in one row simultaneously, the circuitry will produce that row tone without producing either column tone.

Finally, pressing two keys on a diagonal will produce nothing.

Any more modern phone won’t do this. But the old Western Electrics built up through the late 80s will do it every time.

The phone company’s switching system will ignore any single tones it hears, so you can play with this without fear of inadvertantly calling Timbuktu.

[moriah]

@Rayne Man

The reason two tones are used for each number is to prevent false dialling information being sent . If you just had one tone you could , in theory , have the odd background whistle or even the human voice fool the switching equipment and cause mis-dialling. DTMF stands for Duel Tone Multi Frequency.

Stupid Human Tricks I’d Like to See:

Two people singing/whistling/"eeee"ing the right tones together at the right time into a phone handset to dial a specified phone number.

Peace.

[Meeko]

In response to the stupid human tricks… about whistleing the touch tones…

Didn’t Hacker Kevin Mitnick once respond that the Goverment thought he could simply whistle the tones, and how that was not true? (The Screensavers on TechTV/G4 has him saying this)

Also, I hear a few DJ’s state something to the extent of "Listen in for the “Winning Touch Tones” … or Call when you hear the Winning Touch Tones…

Then the “trigger” is a touch tone recording… which I beleive is the exact same recording each time… in fact, are the sounds played corresponding to the Buttons that hold the letters W I N N I N G ? (That is, 2 holds “A B C” etc)

[gotpasswords]

I’ve never heard a radio station spell out a whole long word like that - usually it’s more like their callsign, or a “phonetic” variant - IIRC, 20 years or so ago, WLS in Chicago was using “beep beep beep BEEP beep” for “dub el yew ELL ess” - no idea if the tones actually corresponded to anything, though, or were being used in a strictly musical sense as notes.