Hi,
Here is the code for working with CM8870 DTMF library.
I posted another routine for decoding DTMF from MT8880 Transceiver some days back at http://arduino.cc/forum/index.php/board,22.0.html .
For hardware simply refer to Figure 1. in of the CM8870 IC datasheet.
DTMF.pde (1.24 KB)