Telephone jack and plug

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A telephone jack and plug are the connectors that link a telephone to the building’s phone wires. The plug goes into a jack, which is usually fixed on a wall or baseboard.

Most places now use modular connectors, and the 6P2C style plug is the most common. A standard like RJ11 defines not only the plug’s shape but also which wires carry the voice signal and how the contacts are wired (the pinout).

Modular connectors are used for telephone lines and other services (like Ethernet). Older systems used different connectors, and some call these “RJ” plugs even though the exact wiring may vary.

History notes: phones used to be owned by the phone company and were sometimes wired directly. As portable sets became common, more contacts were needed. Before about 1930, three-contact connectors were common; in the 1930s, four conductors were needed for new circuitry. Bell System developed a four-prong cube plug (type 283) with uneven prongs to prevent misinsertion, later changed to a round version (No. 505A). The four-prong setup stayed common until modular RJ-style jacks became standard in the 1970s.

Nowadays, modular connectors are standard for new installations, though some older types remain in use. A single connector can be wired differently in different countries, so cables aren’t always interchangeable.

For example, the UK often uses a BS 6312 wall plug and a 6P4C or 6P2C plug at the phone end, which may be wired as RJ11 (pins 3 and 4) or as a straight-through (pins 2 and 5). This means you might need different cables depending on the devices you’re connecting.

Typically, U.S. phone cabling uses four wires, but only two are needed for the main line. The other two can carry a second line or power for features like dial lamps.

There are many regional variations, including in Eastern Europe and Israel. Some old or special phones used unique plugs with extra features or testing circuits; many of these are now obsolete or replaced by newer RJ-type connections.