Beware that Openreach will only install the BT socket into the room that supplys the main BT line coming into the house. They will not route wires into other rooms and if you want an extension socket, Openreach will charge extra.
BT and Openreach are two different companies.
It would be the same with any provider. You're not telling me you expected further wiring & a socket to be installed at no cost are you?
not sure why the warning as this has been standard openreach practice. Any wiring after the master socket is responsibility of home owner