I had an SFI come out a couple of weeks ago (totally unnecessary as my ISP and I both knew). Appt was booked for Saturday PM and was a no-show. Engineer turned up Sunday AM. When I pointed out it was booked for Sat PM he said "I know but the system told me Sun AM". Sounded like a right cock-and bull story but at least he turned up.
Unfortunately what we needed was someone from their network team since there was nothing wrong with my line or modem. Well - up till then anyway. The engineer's testing resulted in interleaving being applied and it was two weeks before the DLM relented. All this for an ongoing problem that repeats every 60 days and has done since last June.
Increased latency, sync unchanged, no packet loss. It's the network, Jim! But BT have yet to accept that.