I haven't worked on key systems for many years, and am not familiar with the different KSUs(i.e.,Avaya Partner vs Norstar DS5)and the limitations of their PBX-like features (hybrids).
I was called on a repair for a "crashed" Norstar DS5. It had lost its programming due to a faulty card not keeping the programming when power was lost, and it reverted to factory fault. It also had all DIDs down, and all outgoing lines down in the line pool. I figured these out to be carrier problems. I was impressed that the Norstar handled DIDs. But when I went through the diagnostics and tested, I could not understand how to detect a defective DID card. A simple method would have been green-good/red-defective LEDs, but unfortunately all LEDs are red, and there are no LEDs on DID cards.
Sure-trial and error. roll them. Is there a better way to test for defective DID cards?