"Hidden Interest on paid up accounts??????

I had a payment plan for 2 of my accounts from November last year that should end in May/June. I had paid up both accounts, on one of te account I even received a letter to confirm. Now, when wanting to close the accounts & get an official Settlement Letter from FNB I get told that I still owe a further R2 776 on one and R10 000 on the other due to "interest". Both my monthly statements indicate interest and I had been paying double the payment amounts on the statements. I have asked to see the break down of the statements to show the non accounted for interests to no avail. When I go to the branches they tell me to call the numbers. When I call I wait for a min of 30mins to be transferred from pillar to post and then hung up on. I know how FNB works on this platfrom, they send some automatic response then they never resolve the issue. I can't keep paying & paying. If they get away with this, it means next month they can decide that I owe more. It's enough!!