The phone socket on a BT hub is for a normal analog phone and connects to BTs VoIP service.
Additional analogue phones can be connected using adapters available from BT which plug into a power socket near the phone and communicate with the router over Wi-Fi and also take normal analog phones or as suggested you can use a VoIP adapter such as the grandstream which is what I have, but you would have to get that configured to work with BTS VoIP service unless you are going to use a third party provider.