That is almost the test I was going to suggest. Earth the +ve probe of a DVM and use the -ve probe to test the two wires making up the second pair. If you can see a voltage only on one wire (implying only that one wire is connected) it would be enough to perturb the working pair.
When an xDSL service is operational, the working pair is being used as a radio frequency transmission line. What can be "got away with" at DC and audio frequencies is often detrimental to the good workings of a radio frequency transmission line.
Thank you for those three images. As you say, there is potential for sub-optimal connectivity!
I thought I had an image stored away somewhere but it's currently proving to be elusive.
Time for an update!
I've lost count of the appointments I've had since my last post, one missed... one didn't turn up but put down that he'd been!? and one who turned up without an appointment who put all his stuff out (Ladders, Road signs etc) only to be told not to climb
Which brings us to Friday where a great engineer came out and couldn't understand why this had been thrown back so many times, he climbed the pole... found corrosion in the top of the DP and cleaned that up, was still getting a bridge tap result whilst up there with the tester attached so came down and opened up the pole joint which all looked fine and also showed bridge tap, so he switched us to a spare pair back to the cab which also showed up as Bridged tap!
Eventually another engineer turned up with a Hoist and they went along the line bit by bit back towards the cabinet where they believe they've traced the fault to a 150mtr section underground in the road ducts, unfortunately this requires a heavy lift team to get the covers up in the road, it might also require a road closure!
It's progress, but seems like a really weird fault to have a bridge tap on multiple pairs? The chap in the hoist said they've tried all the other spare pairs and they were worse and some were even showing as going to ground with I guess means they're earth faults?