unplug the problem phone overnight from phone line and any power
adapter and remove its batteries, to discharge any surge problems, and
check it the next day. compare it to a known good phone. phone
telephone repair and have them test each line. try two single lines at
the same problem jack and see if there is crosstalk. check the lines
for a high resistance short with a megohm meter. reverse the line
pairs. reverse the red/green polarity. reduce the phone to one line
use. remove other items on the lines like computer, fax, caller id
boxes.

wrote:
I have very strange problem, and hope someone out there has an idea how
to fix it. I have a 2-line phone, but I get a loud hum on line #2 when
I have both lines plugged into the phone. When just line #1 is
connected, the signal is very clear. I am pretty sure that the problem
is inside, since I've checked each line at the NIC outside, and both
are clear. I have replaced the standard twisted pair wiring with
cat-5, but that hasn't helped. I thought it was the phone and had that
replace, but that didn't fix it, either. Any ideas? Thanks