This is to do with good negotiating practice. Both you and your lender have rights. It is using its rights; you should be using yours. Your first right is to require the lender to account for any claim it is making against you before you do anything about it.
Making offers – even filling in and returning the income and expenditure form the lender will send you – may be taking as an admission that you accept that you “admit” the debt. You don’t. So don’t do it.
You’ll almost certainly find the lender avoids answering the question fully and generally tries to avoid giving you any more information. Yet it will demand quite a lot of information from you. Odd, that! It’s actually just good negotiating technique: give away nothing and find out what you can about your opponent. Lender’s debt collection departments are experts at it, they do it all day.