Thanks for the reply. Unfortunately, I don't get a dial tone when I plug in a wired phone to the test socket I had assumed this was due to the way that orange second line worked. To be clear, I never get a dial tone when I plug in a phone, even when broadband is working perfectly.
I am as puzzled as
Mr Saffron and I will go along with the suggestion that there is a discontinuity in the pair, hence no operating POTS and a degraded broadband service.
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100% Linux and, previously, Unix.