Take a for-instance:
A large rural site I work for, with many incoming lines, just went all SIP.
BT / Openreach couldn't be bothered (literally) to run a leased line when requested. They dragged feet for THREE YEARS. I'm not even kidding. So the order was forcibly cancelled. That woke them up enough to wonder why, because I think they were convinced they were the only ones able to actually get a line there.
Turns out, even though Virgin's business postcode checker "said no", the man on the end of the phone said "Yes" when I asked. We moved heaven and earth and had to do all kinds of things to get it in (including digging our own trench through neighbouring land, etc.) but we got it there. And it's been there three years, speed as promised.
BT / Openreach even tried to access the site AFTER their install was cancelled "to finish connecting us" and I had them removed. Literally, they took three years to put in three bits of empty plastic tubing that weren't even jointed. VM got their line there in 3 months.
So would BT have wanted the work? No, because of one simple reason. In the years of flawless service since, we've ditched every BT line coming into the property and replaced them with a single SIP trunk over the fibre. It's cheaper, easier, allows us to redirect the lines on a whim from a smartphone, can be run over literally ANY internet connection technology we go to in the future, and "just works". Nobody outside could even tell we did it, we still have all the same numbers, better call quality (BT's copper cables collapsed at least once, and every time it rains we lost four specific lines or they went incredibly crackly), and no hassle.
That's what BT don't want people doing, over their own fibre or over their competitor's. They know they'll lose all that easy money from the copper analogue/ADSL/ISDN lines overnight.
I think SIP really worries BT. And I really hope I'm right. Maybe it'll force them to change their ways. I'm still surprised that Virgin Media don't just have a "SIP" option in their default home router (hell, with a Draytek router at home, I can plug in an analogue phone and it becomes a normal SIP client with failover-to-analogue-line if I want - and when unpowered is just a straight analogue phone connection) rather than faffing about with cable-splitters and whatnot like they still do. Surely one device could do cable TV, cable Internet and be an analogue phone interface.
To be honest, workplaces are almost entirely IP and PoE now - phones, wireless, CCTV, etc. I think it's only a matter of time before some small startup pushes it into homes as a commodity technology with some IP/PoE gadget.