What are DSL Phone Filters?

by Steven Melendez

Digital subscriber line phone filters are small adapters that sit between phone jacks and devices like phones and fax machines. They're used to reduce interference between DSL Internet modems and telephone devices used on the same phone lines. The filters are available from many DSL Internet service providers, electronics stores and e-commerce websites and can be ordinarily be installed in a few minutes.

What Do DSL Filters Do?

If you connect to the Internet through a DSL connection, many ISPs suggest that you install DSL filters for every device that connects to the same phone line as the modem.

A DSL filter is a small device about the width of a cigar with a standard phone cord on one end and a standard phone jack on the other end. It keeps signals generated by the DSL modem from reaching other devices on the same line, like telephones, fax machines and caller ID boxes. It also keeps noise from those devices from reaching the modem. Signals in the frequently range used by human voices pass through the filter, while higher-frequency signals are blocked.

Without the filter, signals from the modem can cause line noise and caller ID malfunctions on the other devices, and signals from the other devices can cause Internet connection problems with the modem.

How to Install a DSL Filter

A DSL filter should generally be installed for every device that plugs into a phone jack on the same line as a DSL modem, except for the modem itself.


    • Do not use a DSL filter with the DSL modem itself, since it's designed to filter out signals that the modem needs to operate. 
    • Some home alarm systems that use phone lines to communicate work best with special DSL filters designed specifically for alarms. If you have an alarm system that shares a phone line with your DSL modem, contact your alarm company to find out if you need a special filter.
    • If you have wall-mounted phones installed on a phone jack with no room for a DSL filter, you can buy special wall-mounted DSL filters to use with those phones.

To install a DSL filter for a device like a phone or fax machine, start by unplugging the device's phone cord from the phone jack.

Plug the phone cord DSL filter's own phone cord into the jack.

Plug the device's phone cord into the jack on the DSL filter.

Check for a dial tone on the device to make sure it's properly connected to the phone line.

After all filters are installed, restart your DSL modem by unplugging it from the wall, waiting at least 30 seconds, and plugging it back in. When the modem restarts, verify that your Internet connection is working properly.

About the Author

Steven Melendez is an independent journalist with a background in technology. He has written for a variety of publications and was awarded the Knight Foundation scholarship to Northwestern University's Medill School of Journalism.