By the way, I think you want "MT_D2 = 1;", since from the datasheet's Table 5:

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).