I've also been confused about what the difference is between some people who plug their phones into the FTTP box on the wall and normal copper PSTN line sockets and I'm guessing Digital voice is plugged into the Broadband hub/router.
So will those who connect their phones directly into the FTTP box on the wall have to upgrade too? Is it VoiP or not as some new builds were only provided with this.
In my property they installed the FTTP box and a separate standard master socket for the phone, originally both was with BT, however the FTTP broadband was moved to Vodafone some time ago.
There was an early VOIP system that Openreach provided called Fibre Voice Access (FVA) which required a standard phone to be plugged into the ONT (FTTP box) but this is no longer being provided. I suspect FVA will also be migrated to Digital Voice or other ISP's version of VOIP at some point.