Fire Warden Phone Loop System error detection.

[DeanA]

I am working on an interface unit for a fire warden phone loop system which looks something like this:
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There can be many warden phones in the loop, only 4 units shown above.
There is a requirement for each interface unit to detect any short or open circuit errors in the loop.
I think I have a circuit solution for short circuit detection worked out but am a bit stumped for a solution to open circuit detection so am looking for any suggestions.
For the short circuit isolation and detection I have tested this circuit which seems to be doing the job for the DC power lines.
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If either the +input power or +output power is shorted to minus in or minus out D3 or D4 will rob Q2 of base current and turn off the mosfets Q1A & B.
For the audio line short circuit detection to plus or minus, my idea is to bias it at half rail and use a window detector to make sure it is not above or below certain level.
Any recommendations for open circuit detection?  I’m guessing this problem has been solved for previous safety systems so don’t want to re-invent the wheel.
Thanks in advance for any advice.