In the UK we are lucky to have a bit better of the system - Openreach (or Virgin) own the cables, and ISPs pay Openreach to do the transit. IP based not dark fibre unfortunately.

That said, most ISPs won’t escalate when there’s an OR problem, or at least take a long time to, and then the OR tech is usually just trained to test the cable coming in and not a lot else.

I used to be with Andrew’s and Arnold (run by @revk who surfaces around these parts sometimes!) who were fantastic, because while expensive, the first person who answered the phone understood your summary, trusted you, and would happily beat up OR on your behalf.