Sorry for not replying earlier, I get several hundred emails a day and sometimes messages and postings here get lost in the backlog !
As ALERTLINKS states, the componnts between pins 16 and 17 are there to make the IC less sensitive to short tone bursts. Even though the pairs of tones used in DTMF are chosen because they rarely occur together in a human voice, there is still a small chance of false detections. Those components are there so the IC only reports the tones after they have been present for a certain time so brief sounds are not reported. If you increase the time constant of these parts you can prevent the multiple detections. Inside the IC, when the tones are detected, the "Est" (16) pin respnds immediately but the strobe and data pins are not updated until "ST/GT" (pin 17) reaches the same logic level. The diode is there to make the timing asymetric, a fast response when a digit is detected but a slower response before another one can be reported. The calculations for their values is in the data sheet.
Brian.