Simple Phone Line Coupler
The very simple coupler. It can be used to record audio from the phone line or to play audio to the phone line from a variety of sources. There is no impedance matching, but the circuit will work quite well. The transformer is either one I listed on this site on the Repeat Coil page. The capacitor is any value from 0.1 MFD to 0.33 MFD at 250 volts non polarized.
You need to take phone off hook to hold the telephone line. You can unscrew the transmitter if you have a 500 or 2500 telephone set or just unplug the handset when you want to record without any stray pickup. NOTE: Recording of telephone conversations may be covered by State and Federal laws. If in doubt what you are doing is legal, then do not do it.
Three coils are shown above. (1) The large Western Electric Type 120 Repeat Coil, (2) The small 1:1 Transformer (similar to the TU-016), and (3) A round Loading Coil, which is a 150 Ohm/150 Ohm Repeat Coil, but not designed as one. A House Key is shown for size comparison.