Decoding DTMF Signal using FFT

Given the sampling frequency Fs, I would like to decode the numbers in a dtmf signal using FFT.

The signal was given in an audio format.

I can get the signal from the audio file, find the length of the signal and the total time. I can also plot the signal against its time. All this I can do.

However, I am stuck since I do not know the next step to take.

Help and link to some examples will be highly appreciated.