Currently the landline number is the copper line, wondering if this can be left insitu seeing as FVA has been withdrawn from sale or another solution will be offered?
It can be.
BT often keep the copper line active for voice if 1 is present, though I don't believe they give you a choice on what will be used.
If not the number will be ported to VOIP via BT's new Digital Voice service, which is their replacement for FVA.
If they do choose to cease the copper line and port the number to VOIP then the telephone must be connected to the BT Smart Hub to make/receive calls. There's no other way to use the landline.
The main advantage of FVA was the ability to use your own router while still being able to use the VOIP line via the ONT.
I would expect BT to start using Digital Voice more often where a copper line exists. Not just for FTTP orders but also for ADSL, FTTC and G.Fast.
They put a lot of FTTC and G.Fast lines on to Digital Voice during the trials (SOGEA and SOGFAST) and eventually it will be how they provide voice for all customers.