An ADSL filter is normally a small plastic box with a short lead that plugs into your phone socket and two outputs, one for your ADSL Modem and another for a telephone. Some filters have only one telephone output in them. ADSL filter select the band of frequencies for each of the outputs, phone or ADSL, and send just the correct band to the appropriate socket. The phone output gets only telephone frequencies (from DC to 3.4 kHz) and the ADSL output gets the higher freuquencies well (above 25 kHz).These modular adapters are priced as unconfigured kits. We are more than happy to configure any adapter to fit your application so that they are ready to use upon arrival. Just give us a call and we'll do the rest!