how were DTMF frequencies determined exactly and how can I extend them?
From page 293 of Engineering and Operations in the Bell System (1982-1983) by AT&amp;T Bell Labs:


  The tones have been carefully selected to minimize harmonic interference and the probability that a pair of high and low tones will be simulated by the human voice, thus protecting network control signaling. 


P.S. Yes, I know this answer is 6 years late, I just happened to be researching for the answer and I thought this could help.