I went back and read your 1st remark. You went back to using 2 side by side phones. That is the ultimate test for the lines. If ok, it is the phone, or the cord. Some 2 liners use ac for intercom, and have a conference that ties lines together, or has a flat cord and needs a twisted, for noise and xtalk. They shouldn't interfere any more than 2 different phones. Pairs can be split and xsed with dead wires and pick up noise. Inbalance, and sounds like ground humm, but the 2 seperate phones would be noisy too.
