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.
