Data Protection rules don't allow telecom operators from divulging the info your are looking for and in any case it's immaterial. Unused lines are routinely stopped/disconnected, you can't just carry on using the previous tenants provider anymore.

BT are offering free connection just now, , 18 month contract plus various other Ts & Cs.

Call 0800 800 150 and ask http://www.serviceview.bt.com/list/public/current/Notice09_boo/2-FrameworkImpl337855.htm for New Line Fee Connection Offer. If the first operator knows nothing about it hang up and try again.