Refused refund

I have a new contract with Vodacom they were not deducting their installment. July 27 I went to Vodacom shop to check what the problem is I was told my bank was an issue when I went to my bank everything was OK. I went back to Vodacom shop the lady there called head office they told me they are canceling the debit order I will pay cash. When I arrived home same day I paid using my internet banking. On the 28th July Vodacom deducted the money I have been trying to get the refund. When I call they tell me stories when I called on Tuesday they told me the cash is transferred to next installment. I'm angry because I was not told anything again how can they refuse to refund me and decide to take the cash. I have been using vodacom for more than 10 years but now I'm done.