This module is based on MT8870 DTMF decoder. Its decoder uses digital counting techniques to detect and decode all 16 DTMF tone pairs into a 4-bit code. This module has all the necessary external components including a 3.579545 MHz color burst crystal, timing resistor and timing capacitor.
The package has a connector through which you can connect the module to your cellphone.There must be a valid SIM inside the cellphone. If anyone calls that cell number from another phone, the user has to receive the call and let the caller send DTMF code. The 4-bit output will be received at the pins 'Output 1-4'. When any communication is established, the 'STD' pin will go high.The board has a power switch and a power indicator red LED. When the board gets power, the red LED glows. The signal indicator LED is blue which glows when a signal is received.