The hardest faults to fix are intermittent ones, and usually require multiple interactions between the end-user, ISP support, and BTW/BTOR - quite possibly requiring additional handling between the ISP and BTW to get things escalated.
What you want of your ISP is a support agent who you don't need to re-explain things to, and who doesn't have to pass on the issue to some back-room staff.
The ISP ends up making a key a difference here, by the staff the employ for this role.