Dual Tone Multi Frequencies (DTMF tones) are two different tones at two ends of a spectrum that are used to send information in telephonic communication mediums. The tones represent numbers 0-9 and the symbols * and #. A series of these numbers may be assigned to individuals as points of contact, as in a phone number, or the numbers can be used to relay other sorts of information.
While it may seem confusing, it should be noted that all numbers on a telephone are assigned two DTMF tones, and in all, seven different frequencies are assigned. Some numbers will have the same lower frequency, but different higher frequency, while others will have the same higher frequency but a different lower frequency. The range is between 697 Hz and 941 Hz for the lower frequencies and 1209 Hz and 1633 Hz for the higher frequencies. There are actually eight different tones, with a higher tone for the letters A, B, C, and D as well, though these are not used on a standard telephone.
DTMF is what makes touch tone dialing possible. In the earlier days of numbered telephone systems, the rotary dial was used, and dialing a phone number was accomplished by breaking up a steady tone. The number of breaks in the steady tone indicated the number that was desired. Since then, DTMF tones have made dialing a much faster and simpler process.
The idea behind using two different tones to indicate a number was so that it would be more difficult to accidentally reproduce the sound and get an undesired result. While this may only be a slight inconvenience when dialing a number, it could be a much greater inconvenience when trying to enter personal information into a telephone system. It is impossible for the human voice to represent these tones.
In addition to being a much faster way to dial, DTMF tones also provide another major benefit: they enable the caller to communicate with a computer system using the numbered keypad. Many people are familiar with call systems that assign options to different numbers and ask the caller to press a number to indicate your desired option. In those cases, the system is actually recognizing the frequencies assigned to that specific number or group of numbers.
Now, DTMF tones are being replaced with powerful voice recognition software in many automated answering systems. This enables the user to speak naturally and be recognized, which could be of big benefit to those who have a hard time seeing the telephone keypad. Still, if the system fails to recognize a response, it may also give the option of using the telephone keypad.