So the first thing to say is that even though your exchange area is earmarked for FTTP enablement by that date, it does not automatically mean that each and every property will have access to FTTP, at least not necessarily by that date
Secondly come the withdrawl of PSTN/WLR at the end of 2025 (if Openreach keep to timescales), then in simple terms it means that Openreach are getting out of the voice business. There will no longer be dial-tone on any copper pair / from the master socket. Voice services will be delivered using IP by the nominated communications provider (rather than Openreach). Any analogue phones will need to be connected to an ATA or the analogue voice port on a capable router.
If you still had no FTTP at the property, then your ADSL service would theoretically still have the data access on the line from the exchange, but no dial-tone on the line. The ADSL of today will (eventually) become a product called
SOTAP. To keep using an old analogue phone, as said, the phone would need to be plugged into an ATA that was connected to the ethernet side of the ADSL modem/router or into an analogue voice port on the same. No voice service directly any more from the master socket. The data service over copper remains as is (unless you have FTTP there, and are affected by an FTTP stop sell)