Note - You will need basic electronic skills to build the project in this tutorial. Use this information at your own risk; do not complain if it doesn't work for you. Only use as prescribed.
Telephone technology is pretty amazing. It’s been around for over a century and is extremely backwards compatible. The 1/4" headphone jacks used on musical instruments and high quality audio equipment are identical to the original telephone switchboard plugs and jacks from over a century ago. It’s amazing that just two wires can be used for bi-directional voice (and data) communications and that a century old telephone can be hooked up to modern phone lines.
A hybrid circuit converts the two wire telephone (tip and ring) into the four wires needed for separate audio in / audio out. Okay, you can get away with three wires since you can reuse the ground.
Step 1: Background
Make sure you read that page completely before trying this circuit, some phones are finicky and some tweaking may be required to get this to work.
All you need is a couple of components – a 600 to 600 ohm transformer with a center tap, a 150 ohm resistor (brown green brown), a modular jack, and two 3.5 mm. mini jacks. I mounted my circuit in a wall mount modular jack box.