How to test CenturyLink service at the NID (network phone box)

To find out if an internet or home phone connection problem is inside your home or outside in the CenturyLink network, you can check for active signal at the "network interface device" (NID). 

What (and where) is the network interface device?

The NID is the point where our outside wiring (the provider network) connects to your inside wiring (the subscriber network). Some customers call it the telephone box or network box, while technicians usually call it simply "the NID."


These boxes are usually about 6 by 9 inches and gray (or painted to match the house). It will most often be attached to the outside of the home near the electric meter. Sometimes it is found in the basement or attic. It's usually clearly labeled "Network Interface Device" or "Telephone Network Interface" on the outside.

Phone plugging into filter (diagram)

Network interface device (NID) with "Customer Access" on the side

If you live in multi-unit housing, ask your building manager for help. You may have a test jack inside your unit. DO NOT try to open a network box that serves multiple tenants.
If you have fiber internet service, you have a different kind of terminal. If the box does NOT have a "Customer Access" label on it, STOP. It is likely a fiber network box, which cannot be opened by customers.
Phone plugging into filter (diagram)

Provider access for multi-tenant housing

Phone plugging into filter (diagram)

Provider access for fiber internet (ONT)

How to test service at the NID

To test your network internet or phone service, you will need to bring with you:
  • A flathead screwdriver 
  • If testing internet service: Your modem and modem cables
  • If testing phone service: A standard landline telephone
  • An extension cord (in many cases)


Then follow the steps below:


1. Use a flathead screwdriver to open the "CUSTOMER ACCESS" portion of the telephone network box.

2. Locate the test jack that has a short wire plugged into it. There may be two. Unplug either one of those wires from the test jack.

To see the remaining steps, select the type of service you want to test.

3. If testing internet service: Plug one end of the modem's DSL cord into the test jack and make sure the other end is plugged into the DSL or LINE port on the back of the modem.

4. Plug the modem's power cord firmly into the round power port on the back of the modem, then plug it into an outlet or extension cord. 

5. Wait while your modem boots up, which may take a few minutes. Watch for the DSL light to turn solid green. 

Test results: Is the DSL light green?

YES, the light is green: The problem is most likely with the outside equipment or internet lines belonging to the CenturyLink network. Submit a repair request so that CenturyLink can fix the problem.

NO, the light is off or is red: The issue is most likely with your inside wiring, jacks, equipment, or modem settings. Try these common troubleshooting tips for slow internet. Or check your WiFi, as in many cases the wired internet connection is fine, and it's the WiFi signal that is sluggish.

You can also run Troubleshooter, which can diagnose common problems. 

3. If testing phone service: Plug one end of the telephone cord into the test jack and make sure the other end is plugged firmly into the phone. Plug the phone's power cord into an outlet or extension cord.

4. Listen for a dial tone in the receiver.

Test results: Is there a clear dial tone?

NO, there is no dial tone, or the tone is faint or intermittent: The problem is most likely outside, with our wiring or equipment. Submit a repair request and we will send out a technician. You will not be charged for the visit if the problem is confirmed to be on the CenturyLink side.

YES, there is a clear dial tone at the network box: The problem is most likely inside, with your own wiring or equipment. Use Troubleshooter to get more help walking through steps to check and fix equipment.

Important! Once you've finished testing, don't forget to plug the original test wire back into the test jack (the reverse of step 1). Then be sure to close the box cover and secure the screw tightly.

Need assistance? Contact technical support by phone or chat. We're here to help!

Troubleshoot problems with your home phone or internet service.

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