why deducting yet i've settled the account

I had a cell phone contract with Vodacom that was due for a period of 24 month. Last payment was gonna be on August 2016. Last year December 2015 I terminated the contract and settled the amount due. I faxed all the proof of payments and asked that they convert my number into prepaid. To my surprise last month they deducted R454,99 on my bank account and when I called them they denied and I faxed the bank statement etc still no one was able to clarify why they are *****ing my money. I want it back and to hell with their service I am going to port my number very soon.