Charged for nothing

App**** for a cheque account years ago. I then got 2 cards delivered. I then opted to not bank with fnb and stay with my current bank. I called their call center and the agent told me that I must just cancel the account on the app. I've never used the account. Today I get a call from a debt collection company saying I owe R6500??? I never used the account at all, no salary was deposited in it and I never swipe the card at all.