R6000 Refund request rejected - paid in error

Worste customer service I've ever received. I made a mistake on my banking app and overpaid vodacom, instead of R670 I paid R6700 on the 28th of January. I immediately requested a refund and was sent a rejection saying the money will be used towards my future bill. I had to log another request as this is unacceptable. How do you as a company just decide to keep R6000 that was paid in error. I've been promised Tha money would be paid on the 7th by one of their managers Bongani Radebe and have to date not received my refund. I call almost t every day with no luck. I was then promised it will be in on the 11th but still nothing then they said today the 14th but still nothing. It is disgusting that vodacom staff is so negligent and reluctant to process a request for a refund of R6000 in this day and age. Vodacom should be ashamed of themselves!!!