I am trying to understand an already existing circuit shown below taken from http://boris0.blogspot.fr/2013/09/rotary-dial-for-digital-age.html.
The aim of this circuit for the right part is to produce pwm signal, to filter it and to send dtmf signal back to phone line.
I was asking me if the purpose of the transistor is to adapt impedance of microcontroller output which is between 20 and 50 kOhms and the line which is around 600 Ohms ? Is it doing a kind of sum of this signal with DC of line ? Is it really working like emitter-follower ? (here we are not only taking Vout but Vout + Vce) How to choose R2 value ?