You need to look for Wi-fi calling as an option in your mobile phone setup menu before you go any further. only the more modern phones have the facility.
Some SPs used to do a box which plugged into your router and created a pico-cell around your home so that you could make calls. It didn't matter who supplied your broadband it would still work. However I have a feeling these have largely gone now - look at the Vodafone web site as they certainly did provide them.
One thing has yet to be clarified: if BT is your phone provider but not your broadband, will BT provide a VoIP interface unit that hangs on their phone line or will you be expected to run the BT phone connection through your broadband SP - I'm thinking here of BT v VM as I have?
If you have been able to follow the technicalities of moving to VoIP come 2025 (which is in theory being done so that the presentation to all properties will eventually all be by fibre which needs little or no maintenance) have a look at
www.theregister.com - it is a fascinating read sometimes!