WHAT IS DTMF
DTMF tones are simply two frequencies played simultaneously by a standard home phone/fax or mobile phone. Each key on your telephone's keypad has a unique frequency assigned to it. When any key is pressed on your telephone's keypad the circuit plays the corresponding DTMF tone and sends it to your local exchange for processing.
DTMF tones can be imitated by using a Tone Dialler. It is also possible to record DTMF tones using a tape recorder or computer microphone, then played into the mouthpiece of your telephone to dial numbers. However if there is a significant amount of background sound behind the recorded DTMF tones, the tones may not work properly and cause problems when trying to dial numbers.
Below is a Dual Tone Multi Frequency (DTMF) map for a 4X4-matrix keypad, the map shows each unique frequency which is assigned to each key on a standard 4X4 telephone keypad. The frequencies are exactly the same for a 3X4 Matrix keypad, without the keys A, B, C and D.
Each of the keys A, B, C and D are system tones/codes and are mainly used to configure telephone exchanges or to perform other special functions at an exchange. For example, the corresponding tone/code assigned to the A key is used on some networks to move through various carriers.
When DTMF was created individual and unique frequencies were chosen so that it would be quite easy to design frequency filters, and so that the tones could easily pass through telephone lines (the maximum guaranteed bandwidth for a standard telephone line extends from around 300 Hz to 3.5 kHz). DTMF was not intended for data transfer; it was designed for control signals only.