Dissatisfied by service received

I enquired about an early upgrade in November but I was only eligible in December but retention emailed me and said if I upgrade when I'm eligible il get 3months credited.i did that and December I get debited, I called and complained and I was told I will b credited and it will take 10 to 15 days it's been over 15days. I Called today again to follow up but still nothing has been actioned. Why must I keep calling and doing follow ups when I was promised feedback and action to be taken.frankly I'm tired of begging Mtn to provide me with a service that I pay for...they are so quick to debit you but when it's time to deliver nothing happens I'm tired of this service.