Thank you for an extremely helpful thread, we've been having exactly the same problem.
We have also done many of the tests mentioned in the thread and also received a new old-style hub (exactly the one we already had) to try to solve the problem.
In the last call (this morning), they agreed to send an engineer -- but said we would be levied a charge if it was our fault (!). They offered to send (yet) another hub (the smart hub?), but only if we signed on for them for 18 months. Understandably, I said we had no problems staying with them, but only if our connection did not keep dropping out all the time, so not sure about that.
But now I am starting to think the smart hub might solve the problem. If there is a grace period of a few weeks to check whether the new smart hub solves the problems or we have to find another provider (a pity, because I like the freedom of accessing BT-FON and have been with BT for 13 years), we will go for that.
If you conenction is dropping then I doubt the samrt hub is the solution more like noise on your line
can you post stats from your current hub please
try quiet line test dial 17070 option 2 should be quiet and best with corded phone
are you curently using test socket with filter to eliminate you internal wiring as causing a problem