Hi all, it's me again

I've diagnosed as many things as I possibly can, and can't come up with a way to solve this. Only thing I haven't done is a full reset, and that's only going to happen if someone thinks it'll help.

Incoming Line 2 never makes it to any of the handsets. I've swapped the incoming cable to other lines and get dialtone with the correct phone #. I've also tried other incoming cables into line 2 and still get nothing.

We only have 4 lines coming in, so I can easily use one of the spare 2, but it gets a bit confusing since our main two phone lines in are now split between 1 & 3.

Anyone have an idea on how to solve this other than replacing the KSU (or replacing it and getting it fixed as a spare)?