I first tried to wind my own current sense transformer but I needed more primary turns than the core could hold. I finally used a 115 VAC to 12 VAC transformer. The 12 volt secondary was wired in series with one phone line wire, in order to sense the fluctuating audio currents. The DC resistance of the 12 volt winding was very low so it did not cause problems with the cordless phone operation. The 115 volt primary winding had enough turns to boost the small audio currents to provide .2 to .4 volts DC. This is OK as I was planning to look for 0 volts which occurs with no audio!
I did add a diode between the cathode of the 4.3 volt zener diode and the integrating .1 ufd capacitor and the parallel bleed resistor, to only allow the positive voltage and block the - .7 volts. The capacitor bleed resistor is now 220 K ohms. The bleed resistor is also the input pull down for the Analog input monitoring the audio DC level.
I also put together the On hook voltage sensor monitoring the phone line voltage.
I used a MCT2E opto isolator that I had in my parts bin. I found that it is obsolete but electronics parts warehouses have thousands in stock. Anyway I used its input LED wired in series with a 10 K ohm resistor and ( 2 ) 1N5244, 14 volt .5 watt zener diodes to require a phone line voltage higher than 28 volts DC to turn on the LED in the opto isolator. I also wired a bridge rectifier AC inputs to the phone line wires and the + terminal went to the cathode of one zener diode, the anode connected to the cathode of the next zener, then the anode connected to the anode of the opto LED and the cathode of the opto LED connected to the 10 K ohm resistor and then to the - terminal of the bridge rectifier. I actually wired 4 diodes to form the bridge rectifier. I had a small bridge rectifier, but I also had 4 diodes that I had used in an earlier version that I did not want to waste. The purpose of the bridge rectifier is that the opto voltage sense circuit is polarity sensitive, and not all phone lines are properly wired to provide the same polarity to the wall jack. The bridge rectifier takes care of polarity reversals.
I had a 3.4K ohm resistor before I replaced it with the 10 K ohm resistor. The phone would not ring with approx 5 ma current draw, but with approx 1 ma current draw, the cordless phone does ring. The On hook voltage sensor, does sense that the line voltage is higher than 30 volts by turning on the opto isolator output transistor which pulls down a 10 K ohm pullup resistor. When the cordless phone is off hook the phone line is pulled down to 5 to 8 volts DC. The opto isolator turns off the output transistor, as there is not enough voltage to turn it's LED on. My concern was the opto LED current would be too low to drive the output photo transistor. I was prepared to increase the resistance of the output photo transistor pullup resistor to 100K ohms if it was necessary. So far it is working.