This is a simple sketch that uses the MT8870 DTMF Decoder module with an Arduino.
Obviously, you'll need a MT8870 board. I like this version.
Connect it as follows:
(board) (Arduino)
Q1 <--> Pin #3
Q2 <--> Pin #4
Q3 <--> Pin #5
Q4 <--> Pin #6
STQ <--> Pin #7
Tested on an Arduino Due.