Ok, to answer my own question (I've recently revisited this) you can't use the DTMF command for any oscillator frequencies between 20 and 40MHz. If say I'm using a 32MHz osc and declare that with a suitable DEFINE, PBP appears to assume I'm using a 20MHz osc to base its frequencies around. So, you can still create DTMF tones with say a 32MHz osc, but you need to do it with the FREQOUT command and you need to multiply your chosen frequency parameters/arguments by (20MHz/32Mhz) or 0.625.

Also, I noticed that the frequencies produced from a dual tone creation have non-trivial shadow frequencies in some upper bands even utilising the dual low pass filter suggested in the manual. This doesn't appear to occur with single tones, only with dual tones.

Regards,

Troy