On the whole listen vs test thing.
About 20 years ago, my company got its first internet connection using an ISDN telephone line. After a few months we started having some problems where the connection kept dropping out. The phone company techs kept coming out and throwing their expensive testers on the line, sometimes leaving them to log over several days, and each time the tester said the line was perfect. After about 5 such service calls, I managed to convince the guy that the tester was not telling the truth. He was an older guy and said he would try one more thing. He reached into his bag, and grabbed his old POTS line tester [basically a clip on analog phone handset] he clipped it on, and almost immediately we could both hear the problem. Every once and a while there was a loud thwacking crackle on the line, like a lightning strike. Sure enough after the tech moved the line to a clean pair, we never had another drop-out after that.
Tests are only as good as the test case they are designed to measure. Listening allows for a much broader evaluation, though to lesser precision.