Yip I work at BT
They dont keep customer records but if a line has been there 1 month or 30 years it will show on the systems we use.
It will show as what we call a ceased line meaning its been closed for over 12 months but it will show the names of all the people who have had a line before, there just wont be an account for them any more.
If you can happen to find out what the number is that may help. Or it maybe the advisor could not find the address, sometimes its hard to find one so speak to someone else and have it checked again.
If they cant find anything but your positive its a BT line, the address can be forced through and a charge of £124.99 would be raised but if that is a BT line and no work has to be done then the charge would not be applied when the engineer comes out..
If they find the address on the system and its not showing any previous activity its likely that the bt line is there but no one has ever had bt at the address, its rare but not unheard of but im afraid the charge would still apply as the first person to connect a BT line always has to pay the £124.99 connection charge, so if your gonna do it, dont plan on moving out because when someone else moves in. You have paid the connection and it will be free for the next person