Re: what happens in DTMF
I think what Srikanth1555 is saying is correct although worded strangely. The frequencies are NOT added, in other words you don't get a single tone of 697 + 1209 = 1906Hz. what you do get is a complex waveform which is the sum of the two wave shapes. Consider that one wave may be rising while the other is falling and as they are doing so at different rates and not necessarily in phase, the waveform will be a repeating pattern but not symetrical. Filtering at the DTMF receiver still allows the two tones to be individually detected.
Brian.