DSL phone filters block the DSL signal on a phone line from reaching devices that shouldn't receive the DSL signal. Without the filter, there can be line noise and connection problems. DSL filters require no power to operate, though some high-quality filters contain active transistors designed to refine the signal.
Low-quality DSL phone filters and high-quality active DSL filters have the same basic components, like resistors, ferrite cores and capacitors. The difference between the two is the addition of the transistors in the high-quality systems. Common devices that require the use of the filter are regular phones, cordless phones, fax machines, caller ID units and, in some instances, monitored security systems.