A microfilter is a small device that plugs into your normal BT telephone socket and splits the line into both a standard BT telephone socket and a ADSL broadband socket. You must have one microfilter connected to each BT socket, that has any device connected to it, on the line the ADSL service is enabled for even if you will not connect the ADSL modem / router to that socket. The microfilter must be the first device plugged into the BT socket. Failure to have a microfilter correctly installed on all BT socket you use may result in interference (buzzing noises etc.) on your telephone and / or reduced performance and possible loss of service on your broadband access. If you have a burglar alarm that uses the telephone line it is important not to forget this needs to be connected through a microfilter. We also strongly recommend that you check the correct operation of your burglar alarm after installation of a broadband service on your telephone line. To purchase ADSL microfilters / split