The DTMF decoder is a useful tool used for decoding DTMF generated by telephones. 

http://www.infidigm.net/projects/dtmf2/

The decoded digits are viewed on a 16x2 LCD screen. The DTMF decoder can be directly connected to a Serial port to view the digits in HyperTerminal on a computer. The decoder stores the last 234 received digits in EEPROM. The contents of EEPROM can be viewed on the LCD screen via two scroll buttons. The DTMF decoder has two inputs. A RJ11 jack for connecting to the phone line and a 1/8" audio jack for connecting to a scanner, tape recorder or other audio output device. 

Code is written in C for WinAVR GCC. Complete project is Open-Source. Included is schematics, parts list, pdf to make your own PCB, etc.

Based on the MT8870 DTMF Decoder and uses an AT90S2313