No doubt you're aware, but an IP voice service does not require your end-to-end connection to be 100% digital.
The article @Baldrick1 linked to talks about withdrawal of WLR (Wholesale Line Rental) in favour of IP voice services. It does not talk about ripping out the copper network and moving everybody to optical.
SOGEA/SOTAP runs across the copper loop.
Bob Pullen Plusnet Product Team If I've been helpful then please give thanks ⤵