RJ11

RJ11 is a physical interface often used for terminating telephone wires. It is probably the most familiar of the registered jacks, being used for single line POTS telephone jacks in most homes and offices. more...

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RJ14 is similar, but for a two line telephone jack, and RJ25 is for a three line jack. RJ61 is a similar registered jack for four lines.

RJ11, RJ14, and RJ25 all use six-position modular connectors.

An RJ11 jack uses two of the six positions, so could be wired with a 6P2C (six position, two conductor) variety of modular jack, but such jacks are quite rare. An RJ11 jack is nearly always a 6P4C jack, with four wires running to a central junction box, but unused. The extra connectors used to be used as an 'anti-tinkle' circuit to prevent a pulse-dialing phone from ringing the bell on other extensions. With tone dialing this isn't required so the connectors are used to provide flexibility so the jack can be rewired later as RJ14 or to supply additional power for special uses. Similarly, the cables used to plug telephone terminals into RJ11 jacks frequently are four-wire cables with 6P4C plugs.

In the powered variation, Pins 2 and 5 (black and yellow) carry 24-volt, DC power. While the phone line itself supplies enough power for most telephone terminals, old telephone terminals with incandescent lights in them (such as the classic Western Electric Trimline) need more power than the phone line can supply. Typically, the power on Pins 2 and 5 comes from a transformer plugged into a wall near one jack, supplying power to all of the jacks in the house.

Pinouts

Note that while the old solid color code was well established for pairs 1 and 2, there are several conflicting conventions for pair 3. The colors shown above were taken from a vendor of "silver satin" flat 8-conductor phone cable that claims to be standard. At least one other vendor of flat 8-conductor cable uses the sequence blue, orange, black, red, green, yellow, brown and white/slate.

Read more at Wikipedia.org