RD> The most common problem with multiple lines is bleed-through.
It's called "Cross talk", and I have 4 lines here, and never experienced it.
 RD> The two main causes are 1) the phone company using cheap wire to run
 RD> the second line,
  So, they use good "wire" for the first line, then cheap out on the
second?  No, xtalk is caused by egress from one line too near another.  It
"forces" itself into the other line, like a radio station.
Most of the cases of phone line problems I help solve were either poor/dirty
connections (anywhere in the line up to the interface box), a blown lightning
arrestor (usually located in your interface box), or a cheap phone cable from
the wall jack to the modem.
One way to "hear" how your line sounds is to put a phone on it, flash the
hook once, to lose the dial tone, then just listen.  If you hear a lot of
junk, start looking at connections.  If those are all 100% good/clean, then
it's time to call the TelCo and ask them for a line check.
--gary
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