Single line ADSL filter. This inexpensive adaptor is used to filter the digital noise of your DSL modem from your other telephone equipment. If you have a DSL modem and don't have all your other phone equipment on filters your not going to get the most from your DSL modem! Safe for use with faxes, standard 56K or slower modems, alarm systems, answering machines, and regular phones. DO NOT connect a filter to your DSL modem as it would strip the digital signal from that phone line and you would not be able to use the modem. Use these filters on everything else. The ADSL filter comes by itself.