Just connect the phone in the same way as you would have to the wall phone jack, as we used to do in the 'olden days'.One question - this is connecting to the phone base cable or the receiver to base cable?
I understand that you are not thinking about using this connected to 'TPC' equipment, correct. That would be bad!
I really do not care what your audio source is, I just gave a method to interface an audio signal to the phone so that music could be heard in the Ear piece.
I like the 'retro' aspect of what you are doing, besides I can't hear anything above 4khz anyway. Besides you are probably doing this project for the artistic value rather than as a demonstration of the latest technology.
2 resistors and 2 caps complex???