I've had the same symptoms as you but with a more positive outcome.
For the past few weeks my fibre/ADSL link has suffered intermittent low speed operation, lowest download was 0.02Mbps (as measured by BT speed test at speedtest.btwholesale.com)!
Yesterday the BT technician called and after a couple of hours checking fibre>cabinet>copper>cabinet>copper>house but finding no connection problems, even after changing BT wall socket in house. He was disappointed that he couldn't identify any fault but just as he started to pack his tools, the line suffered noise and the signal speed tanked. He said he'd seen the wiring in the second cabinet was in order but an using older technology. So now myself and neighbours have updated wiring and my speed is back to specification.
I was lucky that my intermittent problem reappeared at the right time so that the technician could provide a fix. We had noticed some occasional noise on telephone calls but didn't link that to such catastrophic data interference. Maybe you have the same symptoms or you cold check by using the 'silent' call number posted by a previous respondent?
Anyway, hope you get a fix to your difficulties.
I'm certainly happy with the service I had from BT. I could see the technician took ownership of my problem was concerned to initially not provide a fix. We were both fortunate that the (probable) cause revealed itself before he left to give us both satisfaction.
