This will be very useful on G120
DSP library that includes FFT http://ics.nxp.com/support/documents/microcontrollers/zip/an10913.zip
DTMF decoding using FFT above http://ics.nxp.com/support/documents/microcontrollers/zip/an10943.zip
then I also found this http://www.nxp.com/documents/application_note/AN10875.pdf
Worth reading if not too busy on a Sunday