Telephone jack gold contacts and wire to the phone get gooey and cross connect on my two line system. Four wire wires are used, seems the clear plastic ones are the worst.
Why do they get goo. Looks like oil, but greenish blue at the contacts?

The Greenish blue or black jack needs to be replaced, draw a diagram or take notes to eleminate another problem.
I also suggest throwning out any Line cords that plugged into it, a bad cord or jack will soon take out the other from corrosion.