Line off — it comes up when you call
Sound check
Each button plays a tone under one more of the conditions a real call adds. Tap them in order and say which one you stop hearing — that one is the bug.
Calls from here are in the log with the recording attached, but they spend nobody's retry attempts and move no card until you say what one meant. Extension 600 is the echo test — it answers, plays your own voice back and is logged nowhere.