There`s not much to this circuit. The two RJ-11 jacks are set up to feed the telephone circuit through from the wall to the phone. The active signal for a single phone is on the red and green wires. Yellow and black are usually used for a second phone line. The 0.22uF capacitor blocks any DC current from flowing through the transformer. The 4.7K resistor limits the current of the 90V ringing signal. The transformer isolates the telephone side of the circuit from the tape recorder side. The zener diodes clamp the 90 volt ringing signal and other transient spikes to protect your recorder. The 10K potentiometer is used to adjust the level to the tape recorder.