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DTMF Tones and Signaling Explained

by Joe DeBari

DTMF replaced rotary dial telephones and has lasted to the present day. Learn more about this signaling technology and how it works.

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You’d be excused for thinking that these letters are what you’d find when you dip your spoon into some alphabet soup, but they actually refer to some important telephone technology! The telephone world is full of these acronyms, and while you may not know what they stand for or how they work, they play a big role in our daily lives.

In this blog post, we’re focusing on one of those acronyms: DTMF, which stands for dual-tone multi-frequency signaling.

Before DTMF: Rotary Dial Phones and Pulse Dialing

At the time of their first arrival in homes, telephones were physically connected to a local telephone exchange building through wires that left a phone’s handset and ended at the exchange. This is what enabled telephone calls to occur. (Check out our blog post on VoIP vs Landline vs Cell Calls if you’d like to learn more.)

These wired connections also ended up having a secondary use: allowing a phone user to directly dial a telephone number from his phone, at first by using a rotary dial on the front face of the phone. This rotary dial had 10 holes arranged in a circle, with each hole corresponding to a specific number—usually, “1” through “9” and followed by a “0.” The dial also had a small metal bar on one side of the circle.

To dial a number, you’d insert your finger into the rotary dial hole for a specific number and pull the dial until your finger hit the metal bar. You’d then move your finger away and the dial would slowly rotate back to its starting position. During this rotation back, the dial mechanism in the phone interrupted the electrical current of the telephone line a specific number of times (depending on which number you selected). The pulses generated by this interruption of the current traversed the telephone wires to the local telephone exchange. There, automatic relays or a human telephone operator decoded the number that you dialed to know where to connect your call. This is known as pulse dialing.

Black rotary dial phone.
A classic piece of communications technology: the rotary dial telephone.

While this was the standard way to dial a phone number for decades, it was cumbersome and had distance limitations. When you needed to make a long-distance call, human intervention—in the form of a telephone operator—was required to connect it.

What Is DTMF and How Does It Work?

In the 1950s, the Bell System, led by AT&T, began research into different transmission methods for dialing phone numbers and connecting phone calls. A method they developed, called MF (multi-frequency) signaling, was so successful in connecting distant telephone exchanges for long-distance calls that they decided to offer a version of it to their customers.

The DTMF Keypad

They called it DTMF signaling. This system replaced the rotary dials on telephones with a keypad that has four columns and four rows. The numbers “1” through “9” are distributed among the first three rows and three columns, with “0” on the second-column, fourth-row key.

On either side of the “0” key in the fourth row are the star (“*”) and pound (“#”) keys. Interestingly, the inventors of DTMF added these particular keys to the keypad because they imagined that telephones would one day be able to access the (then) newly created technology known as “computers” by using these keys.

The fourth column’s keys were given the letters “A,” “B,” “C,” and “D.” These keys ended up being dropped from consumer telephones and are mainly used today on some radio equipment and for internal network control at telephone companies.

Screenshot of the DTMF keypad.
The DTMF keypad. Eight different frequency signals are assigned to the columns and rows of the keypad. (Source)

Signal Pairing

DTMF works by assigning eight different audio frequencies to the rows and columns of the keypad. The columns on the keypad are assigned high-frequency signals, while the rows are assigned low-frequency signals.

When you press a key—which corresponds to a number or symbol—the phone generates a tone that simultaneously combines the high-frequency signal from the column that key is in with the low-frequency signal of the row it’s in. This unique signal pair is then transmitted over telephone wires to the local phone exchange, where the two signals are decoded to determine which numbers you are dialing. So when you press the “5” key on your phone’s keypad, for example, a combined signal tone of 1336 Hz and 770 Hz is sent to the phone company, which then knows that you’ve just pressed “5.” Once they receive the full number that you dialed, they can automatically route your call to it.

Why the signal pair? Turns out that makes it pretty much impossible for the human voice to imitate any key’s DTMF tone. It also helps to prevent interference from being received at the local exchange building by other non-DTMF signal frequencies.

DTMF-based telephones were first offered to the public on November 18, 1963. At the time, AT&T trademarked the name “Touch-Tone” for DTMF technology, so these phones became widely known as Touch-Tone telephones. DTMF was so effective and reliable that it ended up replacing pulse dialing (along with its associated rotary dial telephones) altogether and even transformed the way that telephone companies operated. Because DTMF signals have the same frequency range as the human voice, they can essentially replace the intermediate operators who were necessary for transmitting long-distance calls.

DTMF’s Legacy: How We Still Use It Today

DTMF has survived to the present day as the standard way to make phone calls. Glance at the keypad on your desk phone, or open the keypad on your smartphone, and you’ll immediately recognize the layout as the same one that was created for DTMF technology, minus the fourth column of “A,” “B,” “C,” and “D” keys, of course.

It enabled the development and adoption of other calling services through star codes, also known as vertical service codes. These are the codes you may have dialed for services such as call blocking, caller ID suppression, and speed calling. They consist of dialing the star “*” key followed by two digits, depending on the code that you wanted to use.

And perhaps most importantly, DTMF is also used to interact with many commonplace calling applications that we encounter on a daily basis. Chances are you’ve called a company and heard, “Press 1 for Sales, Press 2 for Support, Press 0 to speak with a representative.” You’ve reached that company’s auto attendant menu, an automated greeting that provides you with menu prompts to reach specific departments or employees. By pressing any of the numbers or symbols on your phone’s keypad, you’re using DTMF signaling to transmit your selection to the attendant menu and get automatically routed to the person/department that you want to reach.

Need to check your work voicemail messages? When you enter your mailbox’s PIN or security code to access your voicemail and then select “1” to play a message, “2” to delete a message, “3” to move the message into a specific subfolder, etc., you’re utilizing DTMF signaling to navigate through your voicemail.

DTMF at Your Business: Free Resources From OnSIP to Create Your Own Auto Attendant

If you need some assistance in creating an auto attendant menu for your business, download our free Business Phone Tree Template. This template helps you craft a professional and user-friendly attendant menu—along with related features such as business hour rules and call failover destinations—that will give your callers a better calling experience. And if you need help in writing an auto attendant greeting script, we’ve got you covered there, too! Check out a variety of attendant menu suggestions in our 10 Sample Call Center Greeting Scripts blog post.

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