24 months contract required 26 payments

i had a 24 months contract with cellc , on the 1st week of the 24 month I phoned cellc to tell them that i won't renew my contract as it ends month end and they told me that I will still have to make a monthly payment. i didn't have a problem with that. In July i got a call from one of the consultants, they told me that it was nice doing business with me as I was a good payer and offered me another contract of which I declined. On the 10th August i got a call from debt collectors telling me I was handed over to them as i owe R800 and something at cellc. I never received any call from cellc notifying me about any outstanding payment. I signed a contract for 24 months and paid for 25 months now they tell me I owe them. Cellc is so ***********. I phoned them on the 10th asking them about the outstanding payment, my contract term and why I was not notified about the outstanding payments, the consultant couldnt give me answers instead she said I will be contacted within 48 hours, still waiting, its more that 36 hours now. i am getting irritating calls from debt collectors and this affects my credit rating because of *********** cellc. i will never recommend any friends and family to have a contract with cellc.