Bad service

I bought an item from Bradlows and paid it off in 6months, the last installment was never deducted from my account and I they can't tell me why they didn't deducted it. So I made an EFT payment to them for the settlement amount. But for some reason they keep on phoning me and say the last installment is in arrears??, This after its been settled. I've bee sending 4 emails to them even phoned which they said they will look into, but still they keep on phoning me non stop