I'll re-word the question slightly, will my copper connection be connected to the ONT or will the master socket remain in place and I'll have both on my wall ?
Whatever happens, they will both remain on the wall.
As for whether the phone master socket remains active, that depends on what service provider you're buying from, and what service you're buying.
In my case: I bought FTTP from Cerberus (data only). I kept my copper phone line and FTTC service from Plusnet, and they were unchanged.
If you buy FTTP from BT, then I *think* these days they will transfer your voice number onto the telephony port on the smart hub, in which case the master socket will no longer be used - but they *could* leave your voice service on the copper. People who have taken the service from them will be able to advise you better.