Dial Tone Decoder

Dial Tone Decoder

Telephone conversations are sometimes surreptitiously taped using microphones or other bugging devices. These devices run the risk of being detected. In some intelligence-gathering tapings, however, the contact telephone number may yield information that is as valuable as the actual conversation. If the content of a conversation is not essential, the contact telephone number can be obtained with a device called a dial tone recorder.

In a touch-tone telephone, each digit from 0 through 9 produces two tones when the particular key is pressed. Each tone has a particular wavelength (i.e., height of the peak and trough of the wave) and a frequency (i.e., the number of waves and troughs per unit area). One of the tones is from a low group, which represents the rows on the telephone keypad. The other tone is from a high group, which represents the columns on the keypad. The function of the dial tone decoder is to decipher the tone pairs and match up the combination with the row and column location on the telephone keypad. In an operating phone, this information is passed to a switch, which routes the signal to the phone line, allowing the call to proceed.

A dial tone decoder is also a standard feature of touch tone telephones, and makes the phone capable of converting the numerical and symbolic information that is entered using the phone's keypad into a signal that can complete the transmission.

A decoder can also detect a busy signal. In espionage, this allows the eavesdropper to find out whether the telephone being monitored is in use. Dial tone decoders can also route the dial tones to a personal computer equipped with an infrared port, as the electrical impulses of the tones can be converted to infrared radiation. Thus, a computer can be used to record the activity of a telephone over time, including the numbers dialed during that period.

Instances of assassination via cellular telephones equipped with a decoder and an explosive device have occurred in contested areas of the Middle East in the late 1990s. When the subject answered the telephone, a code was entered that triggered a blast. Detection of the code by the dial tone decoder triggers the explosive device. In this way, attacks were carried out from remote locations. In an Islamist militant group Hamas attack in July 2002, five Americans and four Israelis were killed at the Hebrew University in Israel after a bomb placed in a backpack in the university cafeteria was remotely detonated by cell phone.

In police investigations, dial tone decoders are routinely used for intelligence gathering, and are also used by telephone repair crews to verify phone numbers.



Goleniewski, Lillian. Telecommunication Essentials. Boston: Addison Wesley Professional, 2001.

Ledwidge, Michael S. Bas Connection. New York: Simon & Schuster, 2001.

Proakis, John G. Digital Communications. New York: McGraw-Hill, 2001.


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