So the old landline services are becoming defunct and we will all have to plug our 'home phone' into the back of the internet router.
This means the landline will only work when the internet is on
And in the event of a powercut will not work at all.
Even when lines go digital it will be possible to use the internet and phone IF you provide a battery backup. BT will provide this for vulnerable customers and probably for those who can't get a cell signal. I think BT will have to provide new routers for most of us.
More info, from BT, here https://business.forums.bt.com/t5/Blogs/The-PSTN-switch-off/ba-p/89705