Terms of contractual agreements with Telkom

I had a 36 months contract with telkom for laptop and router they debited my account for 37 months, then i reversed the installment for the 37 months since the term of agreement was 36 months. they called me for sim upgrade since the current one is expiring. my account keep on increasing with the amount of the previous contract and the existing upgrade. i called them several times seeking clarity concerning the reasons of debiting my after paying for 36 months according to terms of contract but instead of answering my questions, they demanded me to pay monthly for that because i was supposed to cancel the contract even though the terms of agreement has lapsed. now they send their debt collectors to me which i asked them the same questions that they did not answer. since i have paid according to terms of contract my balance keep on increasing every month adding the amount of the previous contract and the current one