Of all the line noise solutions I have seen, this one works the best and
is the easiest to make.  No disassembly of modular jacks is required.  All you
need is a single line wall outlet, a modem, a phone cord with 4 wires in it
(with red, green, black, and yellow insulation), and some cheap parts from Radio
Shack.  All of the electronics parts are standard inventory at Radio Shack, so
you should have no problem getting them...  You know how to solder to do this...
Here are the parts:

1  100 ohm 1/4 watt resistor ->$.19
1  1 micro-Farad (uF) capacitor, 200 volts, NON POLARIZED. (if it has an arrow
   pointing to one of the wires then its polarized) ->$1.99
1  5000 (k) Variable resistor (potentiometer) ->$1.09

        Take the phone cord and cut it clean through about 6 inches from one of
the plugs.  Then strip about an inch or two of the outer shielding on both ends.
Then cut off the black and yellow wires on both sides as they are not needed.
Strip off some of the insulation from the red and green wires on both sides.
Then take the capacitor and solder the two red wires together with one of the
capacitor wires in the middle.  Now, pick up the variable resistor and hold so
that the large post is pointing at y a gun.  The three terminal strips should be
at the bottom.  Now, solder the other end of the capacitor to the center
terminal strip.  Now with the terminal strips still on the bottom and the post
facing you, solder one end of the small resistor to the far right terminal on
the variable resistor.  Then solder the other end of the small resistor inbetween
the green wires like you did with the red ones and the capacitor.  Now put some
electrical tape on it or some shrink tape to insulate everything.  If the wires
touch, the connection won't be lost, but the modem will think so.

        What this does is creat a bridge between the two wires, this filters the
line noise.  You can use this for voice communications, but the difference is
slight.  However, this slight difference can reduce the line noise to the
sensitive modem up to about 98%

Calibration:
        Take the phone cord and hook it up to your modem.  Turn the knob on the
variable resistor all the way to the right.  Then dial a bbs that supports your
max baud rate.  When the carrier tone comes on and the modem should connect (it
won't), slowly turn the know to the left (SLOWLY) until the modem connects.
This should be the optimum setting.  You can turn it a little bit more to the
left...  This really works.

Have fun and happy modeming...