I used to have exactly the same even on ISDN and I believe it to be a local Telephone Company Hardware Problem.
There used to be a myth that there was a "Gain" value that could be increased in the local exchanges to help with the problem. I had something of a long wrangle with the local BT excange manager who disabused me about this misconception. We parted amicably enough with him saying that there was nothing he could do except check the line and that had been done and it was OK...
Soon after things improved remarkably. What a coincidence!
I think that you might get somewhere my repeatedly complaining about the quality of the line. But if you read the BT contract you will see that they make no guarantee about the line quality for
Digital purposes. This point was made strongly to me by the Manager.
But, as said, the line quality can be improved by old-fashioned technical work by chaps on their knees with their heads in big Terminal Boxes by the roadside.
Now, change up to Broadband and these irritations just fly away.
