OpenReach is a private company, and has many loopholes to not support you if they do not want to. If you have a line issue and they cannot find the root cause, they will black list you.
I had an intermittent fault on my line which OpenReach couldn't (wouldn't?) resolve when I was with Zen. Magically after switching to BT, they sent out BT engineer, who escalated to OpenReach, who then found the fault and replaced the dodgy part in the green box.
It might be "firewalled" but that's not my experience. It shouldn't be firewalled, it should be a separate entity. The only reason to bundle it with BT is if BT was getting a benefit from it.
We've had BT/OR announce several times over the past 5 years that they are installing proper fibre in the area to squash competition, and then quietly cancel. Funny how we have an AltNet laying fibre here and suddenly BT/OR are getting their act together to upgrade the area.