Charged after contract + interest

My contract ended, got billed a month later. I called vodacom to explain that I reversed the extra amount and don't mind paying it if they can prove that I owe the money. On the call, the person told me that they cannot see payments beyond 3-6 months (in general for every person) and that they would escalate the matter and get back to me. They never did. I then called again, also went into the store to ask for proof of all my payments so that this can be settled and they logged a report and said that they would get back to me. They never did. Today I see they added R100 as interest. Why? I asked vodacom a month ago several times to prove that I owe one final payment I would definitely pay it if it's what I owe. I contacted my bank who says that there are 23 payments (contract amount) and then a 24th payment to vodacom in july 2015. My contract began on 7 July 2015. I am busy till 3pm each day and have a telkom number now. Please contact me to resolve this so you can stop adding interest.