What a phone jack is and why you might need one
A phone jack is the socket in your wall or on a device where a telephone cord plugs in. It looks like a small rectangular opening, usually plastic, with metal contacts inside. The cord that goes into it is called an RJ11 connector — it's the same size and shape as an ethernet cable but with fewer wires inside.
You need to connect a phone jack if you're setting up a landline phone, a fax machine, or a modem that requires a physical phone line. Even if you use your phone mostly for internet or cell service, a wall jack is where the telephone line enters your home from outside, and you may need to plug something into it.
Key Takeaways
- A phone jack is a wall socket or device port where a telephone cord plugs in, and it connects to the telephone line that enters your home.
- The cord itself is called an RJ11 connector, and it has a small plastic clip that locks into the jack when you push it in firmly.
- To connect a phone jack to a wall, you strip the telephone wire, separate the colored strands, arrange them in the correct order, and push them into a wall jack terminal.
- If you're just plugging a phone into an existing jack, you straightforward insert the RJ11 connector until you hear or feel a click.
- Phone jacks are polarized, meaning they only fit one way — if the connector doesn't slide in smoothly, flip it and try again.
Plugging a phone cord into an existing jack
If you already have a phone jack in your wall and you just need to plug a phone into it, the process takes seconds. Hold the telephone cord so the RJ11 connector — the small plastic plug at the end — faces the jack opening. Push it straight in until you hear or feel a small click. The plastic clip on the connector will lock into place inside the jack.
To unplug it, press the small tab on top of the connector and pull the cord out. Do not yank the cord itself, because that can damage both the connector and the jack. If the connector won't slide in smoothly, the jack may be oriented differently than you expect — flip the connector and try again. Phone jacks are polarized, so they only work one way.
Installing a phone jack in your wall
If you need to install a new phone jack in your wall, you'll be working with the telephone wire that runs through your walls from the point where the line enters your home. This wire is usually four or eight thin colored strands twisted together inside a plastic sheath. You'll need a wall jack kit (available at hardware stores), a wire stripper, and a small screwdriver.
First, strip about one inch of the plastic sheath off the end of the telephone wire using a wire stripper. You'll see four colored wires inside: red, green, yellow, and black. Separate these wires so you can see them clearly. The standard order for a single phone line is red, green, yellow, black — but if you're replacing an existing jack, match the order of the wires that were already there.
Open the wall jack terminal (the small plastic box that comes in the kit). You'll see four small slots labeled with colors or numbers. Insert each wire into its matching slot — red into red, green into green, and so on. Push each wire in firmly until it seats completely. Some jacks have small screws instead of slots; if yours does, loosen each screw, insert the wire under it, and tighten it down. Close the jack cover and test by plugging in a phone cord.
Understanding the four wires and what they do
A standard telephone line uses only two of the four wires inside the cable: red and green. These two wires carry the actual phone signal. Yellow and black are spare wires that sit unused in most home installations. If you're setting up a second phone line in the same wall, you would use yellow and black for that line.
The reason there are four wires is historical — telephone companies ran four-wire cables to every home so that customers could add a second line later without running new cable through the walls. You don't need to understand what each wire does; you only need to match the colors when you install the jack. If the wires are already in place and working, keep them in the same order.
Connecting a phone jack to a modem or router
If you have a modem or router that needs a phone line connection, the process is the same as plugging a phone into a wall jack. Take the telephone cord that came with the device, find the port labeled "phone" or "line" on the back of the modem, and insert the RJ11 connector until it clicks. The modem will now receive the telephone signal it needs to function.
Some older modems have two phone jacks on the back — one labeled "line" and one labeled "phone." The "line" jack is where you plug in the cord that comes from your wall jack. The "phone" jack lets you plug a regular telephone into the modem so you can use both the modem and a phone on the same line. If your modem has both jacks, always plug the wall cord into "line" first.
Troubleshooting a phone jack that won't work
If you plug a phone into a jack and hear no dial tone, the problem could be at the jack itself, in the wall wiring, or at the point where the telephone line enters your home. Start by testing with a different phone cord — the cord itself might be damaged. If a different cord doesn't work, try plugging the same phone into a different jack in your home. If it works elsewhere, the first jack is faulty and may need to be replaced.
If no jacks in your home work, the problem is likely outside your home or at the main junction box where the telephone line enters. Contact your telephone service provider — they can test the line from their end and tell you whether the problem is on their side or yours. If you installed the jack yourself and it's not working, double-check that the wires are in the correct order and seated firmly in their slots.
When to call a professional instead
If you're comfortable opening a wall jack and matching colored wires, you can install a phone jack yourself. But if you're uncertain about which wire is which, or if you need to run new telephone cable through your walls, it's worth calling a technician. Running cable through walls requires knowing where studs and pipes are located, and mistakes can damage your home's structure or electrical system.
You should also call a professional if the telephone line enters your home in a way you don't understand, or if you need to work with the main junction box outside. Telephone companies sometimes charge a service fee for this work, but it's safer than guessing. If you rent your home, check your lease before installing anything — your landlord may require that a professional do the work.
Frequently Asked Questions
Can I plug a phone cord into a computer ethernet jack by mistake?
The connectors look very similar, but they're not the same size. An RJ11 phone connector is slightly smaller than an RJ45 ethernet connector, so it won't fit into an ethernet jack. If you try to force it, you could damage both the connector and the jack. Always check the label on the port before plugging anything in.
Do I need to turn off my phone or modem before plugging in a phone jack?
No. Phone jacks carry very low voltage, so it's safe to plug and unplug them while devices are powered on. You won't damage anything or get hurt. However, if you're working inside a wall jack terminal, it's a good idea to unplug the phone line at the main junction box first, just to be safe.
What if the phone cord is too short to reach the jack?
You can buy a phone cord extension at any hardware or electronics store. It's straightforward a longer cord with RJ11 connectors on both ends. Plug one end into the wall jack and the other into your phone or device. Extensions come in lengths from 6 feet to 50 feet, so you can choose one that fits your space.
Can I use the same wall jack for both a phone and a modem?
Yes, but only one device at a time. If you want to use both a phone and a modem on the same line, plug the wall cord into the modem's "line" jack, then plug a phone into the modem's "phone" jack. The modem will split the signal between the two devices. If you plug both directly into the wall jack, only one will work.