At its most basic, you simply connect whatever was connected to yiur landline to the BT socket on the supplied router. OR VOIP adapter. This will be preconfigured to your provider. That is designed to mimic the electrical characteristics like REN , voltage, ring voltage etc.
All of this is underneath, VOIP.
The only slight difference is how proprietary it is.
Essentially iSPs want to lock you in to their more expensive telephony.
Going to a different provider of VOIP services requires that you do some setting up in the router or VOIP adapter.
Sounds like a plan. I went FTTP because of the unreliability of the ADSL connection, mostly, TBH 5Mbps was actually OK.
I decided to retain my copper wires for now.,
I don't really like any of the options. SIP/VOIP suppliers have realised that SIP services are going to be mandatory for anyone who wants to retain a land line, and have decided to price themselves high.
If it wasn't for the fact that when dialling customer centres I can use a hands free phone on 'speaker' I wouldn't bother with the landline at all.
If I get one call a week that isnt spam or make more than one call a week, it would be unusual.
If they switch off the copper I may well simply do without the landline altogether.