That's why I asked Zorro. I presumed the reason for obscuring the tones is so they can't be decoded but that begs the question 'why?'. The easiest analog method, assuming a standard DTMF transmitter and receiver are being used it to change the clock frequency on them both but if the intention is to hide something, the encryption needs to be more than a frequency shift or inversion. The simpest digital method is to swap the data bits around but all these methods, analog and digital give a 1:1 relationship of input data and output data so they are easy to interpret.
Brian.