Hi Simon,

DTMF signals cannot be decoded when the Call Progress (CP) mode of operation has been selected. Please refer to bit1 (CP/DTMF MODE CONTROL) of the Control Register A Description (Table 5 of datasheet) :
In DTMF mode (logic ‘0’) the device is capable of generating and receiving Dual Tone Multi-Frequency signals. When the CP (Call Progress) mode is selected (logic ‘1’) a 6th order bandpass filter is enabled to allow call progress tones to be detected. Call progress tones which are within the specified bandwidth will be presented at the IRQ/CP pin in rectangular wave format if the IRQ bit has been enabled (b2=1). Also, when the CP mode and BURST mode have both been selected, the transmitter will issue DTMF signals with a burst and pause of 102 ms (typ) duration. This signal duration is twice that obtained from the DTMF transmitter if DTMF mode had been selected. NOTE THAT DTMF SIGNALS CANNOT BE DECODED WHEN THE CP MODE OF OPERATION HAS BEEN SELECTED.

So set the DTMF mode to receive the DTMF tones.

By now,
George