although its not my place to talk about this problem.. I have suffered mucho grief of late which looks like it might have been an intermittent BT line, brought about by small break in the wires. The recent heavy rain and wind made my line bad which helped in showing there was an issue with the line. BT make the threat of £130 call out fee should there be no fault on their line. Oh yes, their office in India is rubbish and told me time after time there is no fault on my line, that was proved wrong.
My line is overhead and the engineer had a trick to expose the fault., maybe as yours is underground records of the weather/test results would help in beating the message home.
BTW: I saw your comment earlier, yes people please note, for many if they are not at work they get NO pay, ("throwing a sickie" is not possible) is it is extra important therefore if appointments are made, full use of that time is made. In my case, I've been awaiting a special tyre, had a day off work for it fitting only to discover at the last minute it was the wrong tyre. The garage was entirely at fault, but I lost a whole day's pay, and then another once they got the correct tyre, that's 2 days pay lost, how would you like that? not funny.
