Took money after contract ended and was cancelled

I wanted to cancel a contract with Cell C and they told me I must do it a month before the end date. I phoned them on the 15 July 2020 as my contract ended the 15 August 2020. My monthly subscription was just over 500 rand a month. I got a statement from them in the begining of August saying I must pay 818 rand. I phoned them why the amount is so high, there response was its because after the 15th the special ends. I told them that my contract was cancelled and ends on the 15 August they said they would esculate it the Ref number is *** and that a consultant will phone me back with in 2 days. I phoned them back after 4 days ans asked about my dispute and the lady said dont worry they will phone you before the deduction comes of my account....well the 818 rand came off today I am so dissapointed in there service. I am going to try and sort it out....guys becareful of cell c