Anyone that’s used a phone knows that there are letters on the keypad that make it handy to spell out words for those not gifted with the ability to remember long strings of numbers. It’s also handy for marketing, for instance 1-800-FLOWERS. Those that still use T9 predictive texting from a digit keypad probably have the letter positions memorized by now. But what you may not know is that there are actually four letters on a telephone dialpad.
Dual-Tone Multi Frequency (DTMF) dialing is the modern way telephones signal the voice network over analog telephone lines. Each keypress is a combination of two specific tones that correspond to the pitch of a key. For instance, the ‘1’ key on a keypad is a combination of 697Hz played in conjunction with 1209Hz. The ‘2’ key uses the same 697Hz signal, but plays is with a 1336Hz tone. The ‘4’ key under the ‘1’ key uses a 770Hz tone in conjunction with the 1209Hz tone. Each DTMF tone is a combination of high-pitched tones and low pitched tones. Normal telephone keypads are laid out like this:
|1209 HZ||1336 HZ||1477 HZ|
You can click on each of those links to listen to the tone they make (Thanks Wikipedia!!!).
The military once used a special kind of phone system known as AutoVon (Thanks to Matthew Norwood for the correction and Jason Schmidt for pointing it out as well). This was a phone system designed to survive a nuclear attack. One of the key differentiators of AutoVon besides being hardened against the Russians was the addition of another column of DTMF keys. These allowed the person dialing the phone to find an open line quickly, or in the event of a full network, to boot users off that were on lower-priority calls. The keys were denoted with the letters A-D and had functions with suspiciously familiar sounding names: Flash Override (A), Flash (B), Immediate (C), and Priority (D). I’m sure most of you networking people out there know where those names are used in our little world. Users that dialed a C before their number could boot those on regular calls or on Priority calls off in the event of line congestion. Flash Override was reserved for use by the President of the United States, as it could boot off anyone on a call. This same kind of preemption capability lives on in CUCM as Multilevel Precedence and Preemption (MLPP). AutoVon was eventually replaced in the 1990s with a newer telephone network for use by the Defense Department. However, the legacy of the additional keys that most of us have never seen lives on.
This is the above table, including the new A-D DTMF tones:
|1209 Hz||1336 Hz||1477 Hz||1633 Hz|
If you are a user of Cisco Unified Communications Manager Express (CUCME), you have access to the AutoVon A-D DTMF tones (from here on out, I’m going to call this “Army Dialing”). The system can replicate the tones from these four keys. You might say, “Cool. What in the hell would I ever use this for? No one can dial these numbers.” Yep. No one can dial these numbers from a regular phone keypad. Think about it like this: you have access to a whole group of numbers that can only be dialed by the people you allow access. The most popular use of this setup is for phone-to-phone intercoms. By restricting the intercom number to an “Army Dial” number, no one can dial that intercom number on accident unless they have a button on their phone that speed dials the number. Here’s an example:
CUCME(config)# ephone-dn 13 CUCME(config-ephone-dn)# number A100 CUCME(config-ephone-dn)# intercom A101 label “Networking Nerd” CUCME(config-ephone-dn)# exit CUCME(config)# ephone-dn 14 CUCME(config-ephone-dn)# number A101 CUCME(config-ephone-dn)# intercom A100 label “Junior Admin” CUCME(config-ephone-dn)# exit CUCME(config)# ephone 2 CUCME(config-ephone)# button 2:13 CUCME(config-ephone)# exit CUCME(config)# ephone 3 CUCME(config-ephone)# button 2:14
This way, my intercom line can only be dialed from a phone with a speed dial button associated with the number. I control who can call me (mwa ha ha…). This could also be used for multicast paging directory numbers. That way, only the designated phones have the ability to page and you can prevent unnecessary chatter on the speakers.
I’m sure if you put your mind to it, you could find all sorts of interesting applications for this kind of feature.
Ever tried playing the “hang up” tone through a PC speaker when others are on the phone in the room? I’m told it makes the others *very* upset….
Interesting article, and I don’t even like voice! 😉 Slight correction though. It was called AUTOVON(Automatic Voice Network). See here: http://en.wikipedia.org/wiki/Autovon
By the time I joined the military, it was already changed over to DSN(Defense Switched Network), but everyone still called it AUTOVON. Next time you and I are out of town together, we can have a nice long chat about military phone networks. Some interesting stuff for sure!
Very interesting — I knew those extra “digits” existed, but never knew what they were for.
One thing (this might be obvious to CUCM people, but all my voice experience is with Asterisk): this would only work if all phones are deskphones where the user can’t control speed-dial entries. If someone hooks up a softphone, couldn’t they enter non-numeric digits just fine? It sounds better suited to preventing fat-fingering than as a security measure. If you actually need an extension to be unreachable except from certain phones, wouldn’t you put that extension in a context only available from those phones (asterisk terminology; hopefully CUCM has a similar concept).