You’ll use a DSL filter when your DSL internet shares the same wall jack (RJ11 copper line) as voice devices like corded phones, cordless base stations, or a fax machine.

• If you hear hiss or buzzing during calls when the modem is on, a filter can help reduce digital noise.
• If your DSL drops or slows when you pick up a phone, filters on voice devices may improve stability.
• Homes with fiber, cable, or a dedicated data jack typically don’t require DSL filters.

Place a filter on each voice device—not on the modem line. If you’re unsure, your internet provider can confirm your setup.

An ADSL filter separates the frequencies used for internet and for voice on a single phone line.

• For your phones/fax: It acts as a low‑pass filter, blocking high‑frequency DSL signals so handsets don’t pick up digital hiss or buzz.
• For your modem: It helps keep voice‑band noise from feeding back into the DSL, which can support a more stable connection.
• Types you’ll see here: Single‑port inline filters for one device, and two‑port splitters with clearly labeled “Phone” and “DSL/Modem” jacks.

Connect the modem to the unfiltered/DSL port (or directly to the wall) and the phone to the filtered port. Actual results can vary with line quality and equipment.

You can shop DSL/ADSL filters right on this category page.

• Search terms that help: “DSL phone line filter,” “ADSL filter,” “RJ11 splitter,” or “single‑line filter.”
• Choose a single‑port filter for one device or a two‑port splitter labeled “Phone” and “DSL/Modem.”
• Pick a pack size (1, 3, 5, etc.) based on how many phones or fax machines you’ll filter.
• Color options like ivory or white are often available to match your jacks and trim.

Availability and fulfillment options such as shipping or pickup can vary by location; check the product page for current options before you order.

1. List your voice devices (corded/cordless phones, fax, answering machine) on the DSL line.
2. For a single‑port inline filter: plug the filter into the wall jack, then plug the phone into the filter.
3. For a two‑port splitter/filter: plug it into the jack, connect the phone to the “Phone” port and the modem to the “DSL/Modem” port.
4. Repeat for each jack with a voice device; the modem should remain unfiltered.
5. Test: confirm clear dial tone and that the modem syncs normally.

These steps can help reduce digital noise, but performance depends on your wiring and equipment. Unplug devices before swapping cables and keep connections snug to reduce intermittent issues.

DSL filters are passive parts and don’t have a set expiration, but they can degrade over time due to line surges, wear, or moisture.

• Possible signs: hiss or crackle on calls, DSL drops when a call starts, or reduced speeds compared with a direct connection.
• Quick check: temporarily connect the modem directly to the wall jack (no filter) to see if stability improves, then replace suspect filters as needed.
• Avoid daisy‑chaining multiple filters on one device.

Replacing older or damaged filters can help restore performance, though results vary with line conditions. If your home has a central splitter installed by your provider, you may need fewer individual filters at the jacks.