The DTMF Shield for Arduino converts DTMF or Touch Tone signals into a format that microcontrollers (uC) or Arduino boards can understand. DTMF tones are analog signals that Arduino and most uC cannot read directly. This module converts those signals into a digital format that uC can easily read.
Here is the functional diagram of the DTMF Shield.
The DTMF Module is specifically designed to plug right into the Arduino boards, but it can also be used with a variety of microcontrollers.