Contract

I took out a contract with Cell C about a year ago, last year I called in to get my outstanding contract balance and I was told by an agent that it was about R2200 and cents so I paid 2300 and my monthly installment is R419, so I advised that I didn't want to cancel but rather pay the outstanding contract amount in advance, so that for the remainder of the term they don't debit from my bank account. I didn't check my statement until today, where I realized I have an outstanding balance of R1300 which will be debited from my bank end of Jan which I fail to understand where it came from since my account balance was fully paid and I was supposed to be in advance for the remaining term... And the payment was made in Sep 2019... When I called in today to enquire about the balance the agent said even if I did pay the commitment amount in full they would still have to debit my account coz my contract has not yet ended which doesn't make sense, when I told her to put me through to her supervisor she hung up on me.