Something occured to me that to get the experience of a old phone I need to feed DTMF tones into the AUX of the Codec. This could also be used to get the dialtone.

I'm thinking of feeding a DAC from LUTs for each tone or superimposed tones, an op-amp adjust it to line-level before feeding it into the codec. I've also read about using PWM to output audio and perhaps with a good filter it can make clean sinewave tones.

This adds more complexity and more things to learn. :)

Then I could change the setting in the codec, if I want to get the tone out to the Modem so the person on the other end also can hear the DTMF tones.

I guess this part will be more relevant after I get the keypad matrix scanning working.