As I was listening to the radio this morning and the DJs were calling up people to tell them they had been shortlisted for some great prize, the following question occured to me:
Do the tones for each number always stay the same and if so wouldn’t it be relatively easy to memorise each number tone and “steal” phone numbers when you have heard them being dialed? Or now with phone banking, “hear” the numbers of people’s credit cards being entered?
No, because in the US, you still have four choices per column, so you’d have to use trial and error to find the right number. For a 16-digit number, that’s a lot of trial-and-error.
As far as the DJs are concerned, many newer telephones don’t even use those tones, or at least, not so they’re audible to the caller. It’s not hard to create a sound effect or two to play to simulate an outgoing telephone call. I’ve done it, and it took me maybe five minutes, including set up and final editing.
Robin
We have the technology
What kind of legal use would this be put to do you imagine?
Not this device specifically but there is/was a portable device to store phone numbers. This could be used when away from home to auto-dial them by playing back the tones into the mouthpiece of a telephone. I guess this could be used in a similar way.
This looks like it’d be for private investigators or police officers to capture phone numbers and credit card numbers. I wouldn’t want to be the poor schmuck who gets caught with one, though.
Robin
Why?
I believe DTMF detectors are perfectly legal (unless a law has been passed that I don’t know about). “Tone decoders” are nothing new. I’ve had several of them, one I got over 20+ years ago and it still works like a champ.
One legit reason for having it is to hook it up to your own phone line and record what numbers are being dialed out. This is far quicker & easier than dealing with the telephone company.
I’m sure there are legitimate reasons for having one, but I’m also fairly sure that, for some uses, it violates various wiretapping laws.
Robin
You can say the same thing about a tape recorder. No, there is no law specifically outlawing the possession of a DTMF decoder.
This is false.
DTMF tones are generated using a combination of two frequencies, one for each column, plus one for each row. Thus, each tone is unique.
Question, though. Possession may not be illegal, but what about using it to mine information from a third party’s phone line without their knowledge or consent?
Robin
Hey, that’s exactly what can be illegal about a tape recorder too! (in some states)
Yes, you’re correct; that would be wiretapping.
Phones use Dual Tone Multi Frequency (DTMF) signals to dial the phone, where each tone is a combination of two different frequencies. The system was developed to signal over (presumably pretty crappy by today’s standards) 50’s long distance lines, decoding the signals with modern technology is pretty trivial.
This is why you should never use a cordless phone for phone banking or entering credit card numbers - you’re broadcasting your numbers to everyone in the neighborhood.
It’s really easy to build a cheap DTMF decoder, the one I have came as a beginner electronics kit from Fry’s. You can also get software that can do it, and there are even people that can do it by ear.
If the DJs were playing the full number over the air, I would think that would be kind of a privacy problem. Hopefully they weren’t broadcasting the whole number, or were playing random tones just to let the audience know they were dialing.
Tone generators were the tools of the trade for the first phone phreaks, Steve Jobs and Steve Wozniak among them. Radio Shack sold them for use with rotary phones.
A question just occured to me that a phone expert might answer: Back in the Stone Age, it was possible to make calls from phones with no dials (to prevent outgoing calls evil laugh) by clicking the receiver button quickly. For example, a 7 required 7 fast clicks, a 5 required 5, etc. That’s how rotary dials worked, anyway. (I guess I was phreaking in the analogue age!) Is it still possible to do that? For backward compatability, do phone-company computers count clicks? If, for example, the 4 button was broken, could a call still be made by clicking the receiver button 4 times?
Sure, which is why reporters have to be careful to notify interview subjects that the call or conversation is being taped, and to respect the wishes of their subjects if they don’t want the call taped.
Given that the site linked to above with the DTMF decoder is aimed at PIs and people interested in “spy” technology, it’s clear that some of its uses may be illegal under current law. And, given that the machine is portable, I’d hate to be stopped with it in my car. At minimum, I’d have some fast talking to do. 
As for the specific tones used, they’re hard for the human ear to distinguish. I can distinguish differences in columns, but not for individual numbers within those columns. So, unless you have particularly awesome hearing, you’d have a hard time decoding numbers without a device.
Robin
Each tone is actually a combination of two tones, one for the row and one for the column. Thus there are really only eight tones and sixteen combinations (twelve of which are used on regular phones.)
If you’re really bored one Sunday afternoon and feel like playing with a telephone, eventually you should be able to discern the two separate tones in each button.
Yes, it’s not particularly difficult to learn to distinguish each button by tone. I’ve known a few people who could do it quite easily, particuarly experienced field technicians in the telecom industry.
A caller at the vet clinic informed me that the tones involved in calling our clinic played “mary had a little lamb.” Someone has waaaaay too much time on their hands.
Sounds like a good hook for the vets to use in a commercial or even in the Yellow Pages.