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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.