I think you will find that even if the previous occupant has terminated his service the line will still have a dial tone. BT don't normally physically disconnect the line now they just disable the service. You will find the line will still be available for emergency 999 calls but nothing else.
We still have a BT line from 8 years ago when we moved here but its not active as I've detailed above. We don't use it as we have Virgins services.
If you can still make calls from the line, then notify BT, because someone's account might be still active and racking up monthly charges. You wouldn't want a demand letter popping in the door, then trying to explain at a later date!.
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