I forget the name of it, but there’s a sort of shared phone line system which splits one line into two, and it causes compatibility issues with DSL
It used to be called a DAX. That was my thought having read the thread. Or – if it’s a remote location – lack of pairs in the DSLAM (Cabinet) or more likely a few very ropey pairs to choose from, none of which are very good.
Not sure if it’s still the case, teh Openreach’s documentation on what’s connected where is terrible. We were once paying them to install some very expensive fibre into a building in the middle of Brum, and the engineers were working with 10 year old annotated drawings!
Anyway, sounds like you have more than good grounds for an escalation/grievance procedure. It was dealing with the finger pointing OR/Provider crap that finally pushed us over the edge to go Cell Broadband.