A DSL filter is used to keep home telephone and DSL Internet signals from interfering with each other, allowing the phone to work correctly. This is necessary in a building where both services are supplied by the telephone company and use the same wires.
A low-frequency device such as a telephone can not process higher DSL frequencies, so an unfiltered phone may not work or may cause interference to a DSL modem. A DSL filter, which is a low-pass filter, allows telephone frequencies to pass through it while restricting the higher DSL signal. These filters are passive devices and do not use power. A filter is never used with a DSL modem.