Recommend viewing original BT6500 publicity which is still on the internet.The base station connects directly to the domestic phone line and secondly into the domestic 13 amp ring main system.  Each of the two slave units connect into the domestic 13 amp ring main.  The ring main is both the source of power and transfers the signal from the base unit to each slave unit.  Supplied with original cables (except the primary cable from the master switch to the base station; 6 rechargeable batteries (do not connect normal batteries).