based on recommendation Atmel AVR-314
Implemented for Arduino mini @8 MHz
Works also @16 MHz, ATMEGA 328 and 168
uses Timer2, hardware PWM pin 3, needs PWM low pass filter (RC)
code and pictures can be found here:
https://github.com/antonmeyer/ArduinoDTMF
works with an old telephone W48, FeTAp 611, Model T29