D-Link DSL-302G Quick Installation Manual page 10

Installing Micro-filters
Micro-filters are obtained from your DSL provider and must be installed on
every normal telephone in the house, apartment or office where you are
installing the DSL modem. Micro-filters ensure that the new DSL data signal
and the normal voice signal do not interfere with each other as they travel on
the same telephone line.
Install a micro-filter on every telephone other than the one used for the DSL
modem. If it's not a DSL modem, it needs a micro-filter.
Appendix