OFCOM require providers to offer methods in access to emergency services where they offer IP based phone services, but it doesn't have to replicate the full service.
https://www.ofcom.org.uk/consultatio...ions-power-cut
Currently where customer's say that require one, Virgin Media offer a battery backup until that will use a SIM card to allow a wired landline handset to dial 999.
The alarm industry is working on moving their services away from connecting to analogue landlines, and towards SIM cards and other updated digital methods -
https://www.bsia.co.uk/blogs/98/bsia...-to-take-acti/