Incorrect billing

In December 2018 I entered into an Explora Price lock contract for 12 months at R749 per month. In June 2019 an additional amount of R35 was added for insurance, bringing the total deduction to R784. In September 2019 an amount of R950.10 was deducted from my account. Numerous phone calls resulted in the same answer: it is an error and it will be corrected. The deduction in October was for the correct amount (R784) but in Nov it was for R707. The last telephonic complaint was on 3 November 2019, ***-i59. Until now I haven't received any reply or explanation from Multichoice. The deduction for Dec 2019 was R907.55. Multichoice still hasn't given me any feedback on why the debit order has changed on a monthly basis, while it was supposed to stay the same for the duration of the contract.