Very fortunately we have some cats, eagles and asbokids here to reveal what is going on !
But unfortunately, even when it has been revealed, ISPs/BT can conveniently ignore it, or only go so far, as the details have been obtained "unofficially".
FTTC is a whole new technology, loosely based upon ADSL2 but using much higher frequencies etc., that is/are much more susceptible to problems caused by line "faults" etc.
The current standard line tests do not often appear to reveal these higher frequency "faults", resulting in LTOK results & a disgruntled end user.
New CIDT tests have been trialed by BT, with a better than expected success rate in detecting/repairing High Resistance type issues that would otherwise be shown as LTOK.
Apparently not all ISPs are planning to use this option though. My ISP (Plusnet) are still only thinking about it.
In the meantime, the response appears to be "Your connection is performing better than its estimated speeds, so sorry, we can't action anything on your behalf".
(I believe we all have strong reservations regarding the accuracy of BT's low estimated speeds)
& That is despite real evidence of deteriorated speeds/stability & records of thousands/millions of various types of error that inevitably end up with a modem re-sync that lowers speeds & adds latency.
Now a dedicated FTTC/FTTH section has been set up, we may start to see a little more traffic, & as users need to be logged in to make postings, hopefully, the anonymous "riff-raff" who post utter rubbish will stay elsewhere.
Paul.