Line Filters are small boxes that fit between your phone socket and your telephone / modem. They ensure that your telephone and modem do not interfere with each other.
Do you need to use Line Filters?
Yes, you must use line filters - ADSL can work without them, but not reliably, and not always. You need a filter/splitter between each non-ADSL device (phone, fax, answering machine, etc), and your telephone line. If your devices are not properly filtered, you may experience line noise on phone calls, internet drop-outs during phone use, or loss of Internet connection.
Line filters can be purchased at any reputable electronics store and are easy to install.