Testing FTTH/P vs copper line network

[borren]

I was recently told that the reason ISPs don't know about customer line faults is that they cannot constantly test their customers lines, if on a copper network, because it takes down the connection during the test (so tests are conducted during the night). However, a fibre backbone network can apparently be monitored in real time for faults so that customers do not have to alert their ISP, as the connection is maintained.