Telephone lines that maybe found in your house may be used not only for making calls but also for other purpose like home automation, remote device control. To make it possible there is an interface between phone line and microcontroller system needed.
This DTMF extension is universal module that can be connected to any microcontroller platform. It detects DTMF tones and decodes them to binary digits that can be easily read by microcontroller, which can decide what tasks to perform. DTMF decoder is based on CM8888 chip which is ideal for interfacing with MCU. Here is an example source code for interfacing with AVR ATmega8535 microcontroller which is written for WinAVR compiler toolset.