Have to draw the line at some point. This is a failure of the service providers selling phone services with no regard for resilience, not Openreach. Plenty of notice was given of changes to voice provision.
You might argue that yes the ISP should perhaps offer a UPS solution for their router, but that still leaves the ONT as a problem. The average end user it would probably never occur to them of the need for a UPS as they never have needed one before. They also don't tend to come in kid-friendly or elderly friendly form factors. The original BBU covered those things perfectly and also had very easy/cheap to replace (and dispose of the old ones) batteries.
I'd certainly argue that it being the ISPs responsibility to keep the ONT powered is not true, the ONT is OR domain and they should be the ones providing something.
I totally get the flexibility of having VoIP over the Internet so you can connect from anywhere, but on the other hand VoIP from the ONT over its own VLAN with a higher priority for its traffic makes a lot of sense for a home phone line. Should contention ever happen on the PON, VoIP should get priority and if its done independently of the Internet connection it also doesn't fall foul of net neutrality.