AIUI SoGEA is just a admin or database tweak, with no technical change to how your services are delivered.
Traditionally the phone number was everything, it was like the main database key. A piece of copper must have a phone number allocated to it before they can do things like "add adsl to this phone number".
SoGEA switches that to the copper pair, so now you don't have to have a phone number before they can allocate other services. That allows things like getting vdsl on a second copper pair you have coming to the house without paying for voice.
I can very much recommend Zen, their customer service has been very good. I've only needed it because of Openreach breaking things ofc, so it will be interesting to see how things go now that I have a Cityfibre last mile connection. So far so good
The router Zen sent me is a FritzBox which can do adsl, vdsl and fibre connections and has the VOIP hardware in it to drive an analogue phone. One thing I never did find out is if it copes if I plug in our extension wiring which used to have 3 analogue phones. Guess I'll never know now I dropped the landline number.
I've seen things on the web showing how to turn an old Raspberry Pi into a VOIP phone exchange, you don't actually have to get your ISP involved in the process.