Quote Originally Posted by lerameur View Post
if I choose to do a low pass filter, what would you suggest my cut off frequency be?
Well, since DTMF freq's run from 697Hz to 1633Hz (reference Wikipedia), shooting for 1634 would be a good bet, but probably not practical. I'd tend to think that a 'brick wall' above 2KHz would do it.
And I'd also be willing to bet that the 'air' you are hearing is in fact aliasing of the fundamental tone due to the numerous harmonics caused by the square wave output of the PIC.