On Networks N300RM Installation Guide - Manual (Page 5)

3
Install Your Modem Router
A microfilter filters DSL interference out of standard phone equipment
that shares the line with DSL service. Examples of devices are
telephones, fax machines, answering machines, and caller ID
displays.
1. Connect the ADSL.
ADSL Phone
Line 1
a. Install an ADSL filter/splitter between the phone line and the
phone.
b. Connect the ADSL port of the modem router to the ADSL
port of the filter/splitter.
c. If your modem router and telephone connect to the same
phone line, use an ADSL filter/splitter for every phone line in
the house.