Telephone and RJ11 Cables

Telling RJ11, RJ9 and RJ45 plugs apart

RJ11 is the slim modular plug on landline phones, fax machines and DSL modems, usually wired as 6P4C or 6P2C: six slots with four or two metal contacts. RJ9 is the smaller 4P4C plug used on handset and headset cords. RJ45 is wider, with eight contacts, and belongs to Ethernet, so an RJ11 plug will physically enter an RJ45 jack and can bend its outer pins. Most voice line cords are reverse wired, which ordinary phones and modems tolerate because the line pair stays in the center, but some office systems expect a straight cord, so replace like with like.

Flat or round cord, and how far the line runs

Flat silver-satin cords suit a short hop from the wall to a phone. For a DSL modem, a round twisted-pair cord resists noise better than a long flat one, and the run between the wall socket and the modem or filter should be as short as practical. Longer cords of 25 ft or 50 ft are fine for plain voice lines, and an inline coupler joins two shorter cords.

Headset cables are the exception to all of this. Manufacturer-specific quick-disconnect and RJ9 cables fit only the headsets and phone adapters they were built for, so match the model named in the title. Some listings here are USB or charger cables instead of telephone cords, so look for RJ11, RJ9 or a contact code such as 6P4C in the title.

Questions about Telephone and RJ11 Cables

Can I use a phone cable as an Ethernet cable?

No. An RJ11 cable has two or four conductors and is not made to Ethernet standards, while an Ethernet cable has eight wires in four twisted pairs. The narrower plug also fits into an RJ45 socket and can bend the outer contacts, so do not force it.

What does 6P4C mean?

It means six positions and four contacts: the plug body has six slots but only four carry metal contacts. A 6P4C plug works on standard single-line phones and DSL modems, which only use the middle pair of wires.

Do I need a special cable for DSL?

A standard RJ11 line cord works. Keep it short, prefer a round twisted-pair cable over a long flat one, and avoid coiling the excess next to power adapters, which can add noise to the line.