I wasn't aware of this — I don't live in Britain, and wouldn't use a landline if I did — but I will check my parents aren't worried about it. It looks like their existing DECT phone will just plug in to the router.
There are plenty of areas without real landlines already. Maintaining aging copper infrastructure simply isn't worth the money, so your "landline" is now a SIP connection turned into analog by your modem.
https://www.which.co.uk/reviews/broadband/article/digital-vo...