Dirty Phone Lines

[Dave Hughes]

How can I deal with a chronic problem of very noisy incoming phone lines
to a system from rural analog phones? Users in rural areas are dialing
in at the rate of over 50 calls a day, over ordinary phone lines, which
sometimes are so noisy that they write binary garbage into name/password
lines, freeze the output, or even disconnect. There is no option to
fix the lines at the other end, only between the downtown switch and
the system, and maybe in the system. 
   The lines are coming through Robotics 2400 and 1200 baud modems, a
Computone board, into the system. Would there be any real difference if
we changed boards to non-intelligent serial boards. (Is an 8 port
Digiboard non-intelligent?).
  Is there any good hardware/software solution at the system to filter
the signal better?