Dynalink C10 ADSL2+ MicroFilter

The Dynalink ADSL 2+ microfilter performs two main tasks. First, it prevents high frequency noise from appearing at the telephone and potentially decreasing voice quality. Second, it prevents the telephone equipment from interfering with the modem by preventing harmonics, overtones and changes in impedance caused by the telephone equipment.

Your ADSL modem does not require a filter. It will connect directly to any telephone jackpoint.
Failure to connect ALL non-ADSL equipment via a filter may result in loss of the data link whenever a call is made or answered. In many cases the link will also be lost when a call is received, even if it is not answered.