Customer services officer is under which dept?

This is usually a position in banking. It is mainly in front line banking, such as the receptionist and specialist sales responsible for selling banking products like credit card, home insurance...etc. It is also possible for someone with this title to work in tele-sales, servicing customers in general through telephone but also cross-sell other products.

what kind of company???

there is customer services department mainly in big company.

e.g.
mainly handling: enquiry and complaint
front line staff at reception in bank

etc