It is perhaps understandable because generally people who still have multiple home phones nowadays have DECT handsets, not wired extensions to individual rooms.
Also there are ‘reinjection’ faceplate kits available online that allow a connection from the Hub Telephone socket back into your old master socket to connect to the extension wiring. If you really want to keep your old wiring this seems to be the best option. Note there are different versions depending on whether you will be on a copper SOGEA connection or on a new FTTP connection.
I have just had full fibre installed, and have a phone connected by DVA, and one plugged into the SmartHub2 phone socket. Why can't we share a phone call on both phones ?
Please don't post the same question on more than one thread, it just causes confusion. I gave answered on the other thread.