I've had line tests since this error came up., I don't know if these tests were the same as the one the engineer who found this fault carried out.
He had come across it once before and he said it was resolved by refitting the cable that goes into the ONT from outside - apparently, it was the wrong way in the previous install he found the issue but on mine, it was correctly installed.
He had to call Sheffield and had the line "rebuilt", retested and all was fine.
My FTTP was a new build for the town and I was the first to have FTTP on the pole. The ONT is ADTRAN if that offers any insight.
Edited by Ahmedg (Wed 17-Apr-24 14:23:07)