That's done sometimes in central offices, although for analog lines a lineman's handset was the more common tool.
Digital test systems (I don't know what they use now; back then the venerable T-BERD 224 was the standard tool) can decode a single DS0 out of a larger multiplexed circuit and play the audio back and usually allow you to insert audio into a channel. That's normally what was being used to isolate a fault at one or more of the mux/demux/translation points.
Digital test systems (I don't know what they use now; back then the venerable T-BERD 224 was the standard tool) can decode a single DS0 out of a larger multiplexed circuit and play the audio back and usually allow you to insert audio into a channel. That's normally what was being used to isolate a fault at one or more of the mux/demux/translation points.