I thought I knew what I had, but reading this lot leaves me very confused!
A while ago I had a long running battle with BT about the fact that my Speed had dropped to zero. (from about 4 - 6 mbps)
Lots of conversations with lots of Indian call centre workers, remote operation of my pc and various drop-in engineers got me nowhere.
Letter to BT CEO, and letter to mate who works in BT got someone's attention, and the delights of an English-speaking "case manager" who promised to stay with my case until it was resolved. (Which he did).
The drop-in engineers changed to another breed of drop-in engineer, (one of whom told interesting BT stories about the BT hierarchy and its broadband connections). This new breed said they knew more about broadband than the other ones.
One of them put in an "Openreach ADSL v1.0" socket about 24 inches from where the main BT line pops out of the ground, and said that thereafter I would not need ADSL filters anywhere in the house anymore.
That, plus lots of other mucking about by BT engineers, both here, and at the 1km away cabinet, and at the 1.8km main BT "shed" got me back to my old 4-6.
..........but this thread has made me terribly confused.