You must remember, with all these OFCOM rules, that the left hand doesn't know what the right hand is doing.
For non full LLU stuff.
The phone provider doesn't know the ISP, & the ISP doesn't know the phone provider, only that the phone provider using BT wholesale equipment.
As far as I know, neither, BT wholesale, or Openreach is allowed to tell either of them.
Normally, it's not a problem, but change a phone number, & there's a records mismatch.
As one side can't tell the other, you can end up with problems.
As the customer is the only who can talk to either side, it's in their interest to keep both informed.
plusnet Fibre Unlimited
Using a Draytek Vigor 2850Vn