I am both a BT and Virgin Media customer having my current analogue landline (POTS) with BT and my broadband with VM.
In the BT Digital Voice VoIP scenario how will my landline work?
As a more general question, how will the switchover work for customers who have analogue handsets but no broadband connection? I'm thinking specifically of older family members who do not currently embrace such technologies.
BT would probably provide you with a suitable interface box using a low speed broadband connection.
Read this two page website.
https://www.draytek.co.uk/information/blog/the-end-of-analogue-phone-lines-pt1