The line modules do fail quite often, particularly during thunder and lightning!
I have given up installing these systems as have seen too many go belly up during/after storms. Even if it appears just the line card has failed, you often see further issues days later. I've replaced so many systems!
You need to be wary with second user systems as so many have been removed after storm damage.
I'm no longer a fan of these
XN120,SL1100 & sv8100 are far superior and just never seem to fail! Touch wood!
I'm assuming you're using a proper punch down tool for the connections?
If you're using standard PSTN lines (via NTE5a's), you will likely need to alter the guarded clear setting too, otherwise the system hangs on to the lines to long after a call has cleared down.