Vodacom fails to refund me on incorrect charge of R12111.62

I renewed and upgraded my contract with Vodacom through iStore. Included in my contract is payment for my cell phone. However Vodacom billed me and my bank was debited with an amount of R12111.62 on 31 May 2018. I pointed this out to Vodacom immediately. I was told that I would be refunded. Instead of a refund to my bank account my Vodacom account was credited. They have said that it would be paid into my account. To date it hasn't been paid. I have called them over seven times.