Old rotary-style phones were equipped with a cable that wired directly into a small phone box on the wall. When you purchased your phones from the phone company, they would come out and install them. Later, you were able to purchase a large, four-prong plug that connected into a four-prong outlet. This allowed the phone to be unplugged and moved to a different location in the house. Through miniaturization, the modern modular jack was invented. To make a rotary phone plug into a modular jack, you need to change the cable.
Installing a Modular Plug on a Rotary Phone
Remove the cover of the rotary phone. Turn the phone upside-down and loosen the two screws holding the cover on. One will be in the middle front and the other in the middle back. The screws should remain in the phone — only loosened from the cover.
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Locate the wire coming into the phone. There should be four wires — red, yellow, black and green — attached to four screws using U-shaped blade connectors. Loosen the four screws and remove the four wires, noting the location of the colors.
Prepare your modular wire. The best type of modular wire is one with a modular jack on one end and four U-shaped blade connectors on the other. But these may be hard to locate. Next best is a thick modular wire that can be stripped.
Connect the new modular phone wire to the screws. The only wires that are used for normal calls are the red and green wires. Connect the red and green wires to the appropriate screws inside the phone.
Replace the cover on the phone and tighten the screws.