Notice: At one time our telephone service providers sent routing control information within the signals used for communication. This in-band signaling allowed the theft of their services by people who knew the proper frequencies and ways to use them. All of the major carriers have now moved their control information out of the standard band so that such theft is no longer possible. In any case, the information given below is for the standard tones, not for control, and is freely available from the comunications companies.
Dual-tone multi-frequency (DTMF) signaling devices generate two separate tones for each key pressed. With the keys in the standard Touch-Tone™ rectangular array, you can read the frequency at the top of the key's column and at the left of its row to see what is sent, or you could click on a key to have that information displayed directly. The operator's telephone has four extra keys down the right edge to use for directory assistance. All frequencies are given in Hertz.
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