The D-Link DSL-15MF is a Microfilter (or Splitter) and is used on a line or extension where both an ADSL modem as well as a PSTN device is desired. The unit splits the line, providing a pass-through connection with no filtering to the ADSL modem, while providing a filtered connection to the PSTN device. Note that all other extensions still need to have a filter of some kind placed on them.