Did this setup used to work and now its having the problems? This may be totally off the wall, but the co lines are connected to each phone with 2 line cords, one for co 1-2 and the other for co 3-4. If someone was unplugging and plugging cords in at a phone and happened to get the cords plugged in incorrectly, you'd have a ton of crosstalk............I've had this happen before.
Just an idea.
