"Internet goes down, so does your phone line; emergency calls, how?"
Mobiles were working long after BT landlines stopped working in terms of Grenfell (as an example).
Mobile networks have to be just as resilient today, in terms of emergency services, to withstand multiple points of failure before they fail completely. (There have been quite a few large headline fines for failure in this regard). Maybe not yet as good as landlines yet, but 7 years from now?
Add another 7 years to BT's investment in EE, you can see where this is going (current Government investment in Emergency services 4G network via EE/BT), in terms of mobile resilience.
I'd say traditional/VoIP voice landlines are dead. Yep, they're dead Jim. Not sure why a BT VoIP service is even being discussed as such a necessity. Habits are changing fast.
10 years from now, the cabling to most homes will be for full fibre services, whether by Vodafone, Gigaclear, CityFibre, B4RN or if BT pulls their finger out, BT (+ others).
This is the start of the legacy copper carcass switch off. It's far less reliable than you think anyhow in terms of disasters outside of bomb proof bunker type Nortel exchanges metres below ground in City of London (which were installed en-masse after 1993 Bishopsgate's bomb)