# Does each phone number have a unique tone?

I spend way too much time in my car listening to news and talk radio and over time something has piqued my curiousity. I have noticed that on talk radio the hosts often dial the phone number for the next guest they will have on the show. You hear the characteristic do-do-da-de-do of the phone being dialed.

I do not have a trained ear, in fact I am just about completely tone deaf, but each number sounds like it has a distinct tone. First, is this correct? Is there a different sound for one vs. two on a telephone. Second, can someone with a trained ear distinguish and thus discern the phone number by identifying the distinct sound of each number?

I would imagine that, if my premise is correct, there must be equpment or a computer program that can perform the task of distinguishing the numbers even if a person can’t. If this is true then numbers dialed over the radio are not really private, it just takes a bit of work to figure them out. Can anyone shed some light on how this works?

Yes. Every touch tone is distinct. It is a combination of TWO specific frequencies.
If you listen to your touch tone phone you can hear the differences. (Ever play “Mary Had a Little Lamb” on the phone?) A little more difficult to discern are the secondary frequencies. Press 3,6,9, and the pound sign on your phone. You will hear that the louder tone stays the same, but the underlying tone is different.

Incidentally, there are specifications for another four touch tones, but these aren’t found on most phones. These are basically tones to the right of the 3,6,9, and pound.

Oh, and to answer the rest of your OP, yes, it is possible to figure out the phone number from the tones. It’s fairly easy to do by computer, especially since the tones are standardized. I’ve also known several musicians who could do it by ear.

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.

Yes, there are machines that can distiguish the tones and dial the numbers. They’re called telephones.
You don’t need to press the buttons at the dial tone to make a call. If you hold a speaker up to the line and play the right sounds, the number will be dialed. A company was marketing a little device for kids. If something ever happened, all they had to do was put this little hand held thing up to the receiver and press a button. The device would play the tone sequence for their home phone to call their parents. I forget the specifics. Thinking about it right now, it sounds like it’s worthless. Why can’t the child remember his number? He will have to have some change on him. He’ll have to make sure he gets lost with this device. Maybe that’s why I only saw the thing once briefly many years ago. I’m sure it flopped!

Incidentally, the MJ from the MJ Morning Show never plays the full number on the radio for this specific reason. You’ll hear the first 3 or 4 while he’s dialing, then he mutes the speaker or whatever so we can’t hear the rest. He’s mentioned a couple times that he does it for privacy reasons and admits that he can personally distinguish the tones and could tell you what number you dialed by listening to it.

Yeah… My Dad is a gadget freak, but rarely uses any of the gadgets people buy for him. One of the few gadgets I bought for him over the years that he really used was an organizer\DTMF generator. They used to sell these back in the late 80s or early 90s. It had a primitive phone book that stored like 300 numbers. You’d scroll through the list and when you found the number you wanted, you’d press the speaker up to the phone and press the CALL button.

There used to be a program called CoolEdit (now called Adobe Audition) that generates DTMF tones. I used it to generate MP3 files of some of the longer strings of numbers I’d have to dial (such as my voicemail and also programming remote call forwarding to route calls to my cell phone). These strings were longer than the 20 digits my cell phone could hold as a phone book entry , so I copied the MP3s to my iPaq. I’d then call the number, press the iPaq speaker close to the phone and play the MP3.

D’oh! I should have pointed out that - this being the late 80s and early 90s - cell phones didn’t really have much in the way of “phone books” yet and Palm Pilots were still a couple of years away. So Dad’s dialer device wasn’t as dorky a gift then as it seems now.

Nitpick:
Yes, however, it is the telephone switch that does the decoding, not the telephone itself. Telephone are in general pretty stupid, and just generate the standard tones when you push the buttons.

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.

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.

You might also be interested in reading about phone phreaking – ways to hack into the phone system, often by using tones other than the standard ones to fool the switching circuitry.

(For example, there’s the famous John Draper, aka “Captain Crunch” who discovered that he could use a whistle out of a box of Captain Crunch to fool the phone system into giving him free calls … .)

It should be noted that most of the “phone phreaking” stuff like blue boxes doesn’t work in most places anymore (If you dig into the previous link, it mentions this). In its heydey, most of the country was on crossbar switches which carried voice and DTMF signalling on the same lines. With the move to digital switches, inter-office signalling was carried on a separate line to support something called “common channel signaling”. This wasn’t done explicitly to defeat phreakers, it was simply a design improvement in the way switches communicated, but it had that effect.

Draper is still around - an interesting character, and not shy about self-promotion:

http://www.webcrunchers.com/crunch/

Well maybe they’ve debugged the national phone system, but they still haven’t fixed some phone answering machines. I have the unwanted talent of being able to generate the perfect tone to trigger assorted functions on some of my friends’ phone machines. I’m not a great message leaver, so I’ll say a lot of ahhh’s and ummm’s. Sure enough, the machine will think it’s being remotely cued and start doing all sorts of unwanted things. One friend disconnected his machine because it happened so often when I called.

The ironic thing about me being such an uncanny tone generator is that I am so utterly, pathetically tone deaf! I can’t tell a high note from a low one, let alone reproduce it on demand. (No, you don’t want me to be around on your birthday, trust me!)

Or as it was once described to me: “Dial Tone, Mother F#@%er”.

Rex Fenestrarum wrote

Hey I still have one of those somewhere. It was super-cool in it’s day.

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.

at one time they had to convert all the public phone booths here [Public Call Office in British Post Office parlance] back to loop disconnect signalling because of people duplicating the tones to make free calls. They are now MF again. There’s a constant battle between the scammers and the fraud investigation unit.

Back in the days when cell phones weren’t particularly common, people used calling cards a lot more often, particularly from payphones. I was once told that I should look out for individuals loitering near blocks of payphones (this was in Grand Central Terminal) who could, by listening closely, hear the calling card number and PIN as you typed them into the phone. They would then sell these to people who would ring of hundreds of dollars of international calls on your card.

It is entirely possible that this was urban legend, but I don’t see why it wouldn’t be possible, with a little training.

Dual tone would make more sense. Typo?

Your are correct. My spell-checker missed that one ! :smack:

But it’s so much easier for them to watch your fingers as you punch in the numbers. Most folk who are calling have to read the number off the card, and that’s darned easy to watch.