DTMF the world standard for music listening that the digits on the keyboard the phone shows . With phone, cable, ground button ( touch - tone ), but pressing a key on the number pad., the sound of the DTMF corresponding to those keys it creates . So the system, phone, cable, Terrestrial, could be it will hear the sound and , and thereby determine the which key is pressed to dial .
mobile networks to direct dial instead of DTMF from digital signals they use, but DTMF is still in the mobile routing system, automatic like browse the phone and dial for the secondary, such as the use of the phone card is used . Each track DTMF in fact, two tracks - a song low frequency and a song of high frequency have been combined . ( So from the direction of the multi-frequency song of the Dual is called ) . If the standard keyboard, the phone as a network plaid look and the sound of the bottom row and the sound of a high column, it can be.