Accuracy of DTMF signal
All dial tones generated by the MCU and the DACs can be always detected by the decoder correctly even though the waveform shown on the scope seemed to have little noise. We never got any mis-decoded digit from the decoder.
Robustness of the whole device
The e-directory works perfectly as what we have specified. All the buttons were debounced and the string pointers were handled correctly such that no character is ever shown on the LCD.