(A) A common surplus AC brute force line filter will do a
splendid job
of removing RF from your house phone lines. Place the line
side towards
the NIC and the load side towards the phone instrument/s.
Ground the
metal case.
I don't think so.
Line filters, since they are designed to pass only 60 Hz,
generally employ large capacitors. They will unbalance the
telco lines and load the lines down....and almost always
have much less RF impedance than even the poorest RF
filters.
The best solution is to use two RF chokes, one in each wire,
of 1000uH or higher. Place a cap across the line at the
input and output, but don't go bigger than .001uF (1000pF )
and be sure the cap handles at least 1kV.