The problem with the ISPs taking ownership of the communication with the ISP is that it overlooks the problem that not only are Openreach ruddy useless at communication, or even prioritising their own workloads, meeting their own SLAs etc - they don't care about the ISP's SLAs either. So quite often issues drag on forever not because the ISP's doing nothing, but because the ISP simply cannot get Openreach to do their job.
In the past I've had BTOR stall an internet provision by 3 months because of a work order they failed to action a whole year earlier - a failure to deliver that ensured all properties connected to the same cabinet incapable of getting any internet at all - you'd never guess what their solution was. Put it this way - they instructed BT Retail to tell any customers registering for broadband off that street cabinet that they all lived in a notspot and couldn't get it. Fortunately for me, I was with a different ISP at the time and they knew different, and they supported me when I called bollox on Openreach's bull. Reported it to the CS director of BT, and within a week the engineers were doing the job they'd ignored for over 15 months.
Currently experiencing 3 weeks of no internet after a house move where BTOR swears blind they've already activated FTTC to my line and not fouled it up, but an availability check on the BTOR systems tells me my West Yorkshire phone number is connected to a cabinet near to its exchange in a rural town in Scotland, and multiple known-working test devices at my end are telling me there is no VDSL or DSL chatter detectable at all on the line. I managed to collar an Openreach engineer at the cabinet, and I got the usual "don't know anything at all about that side of the business guv, nothing on my job sheet here" brush-off.