Double billing - CellC refuses to refund

I signed a contract for R199 p.m with CellC Cresta in April 2017. They then decided to bill to double bill me the month after I signed the contract. Initially I was told that it was an error made by the store because they did not successfully cap my data usage after I had requested that they do.

I was promised a refund and have been following up until September 2017. When the refund was finally due for payment, they changed their story and said that a call was made to increase the limit on the contract. This is not true, and I have not been provided with a telephonic recording of this 'call'!