Unfair treatment

I bought an IPhone 6s at Mtn Maponya mall in July 2017, beginning of September my phone stopped charging,I took it back to the shops,they booked it in for repairs and told me it will take 4-6weeks,after six weeks I called them they told me that it’s not back they still fixing it,I waited until month end October I called again they told me that the phone is back but now they saw that it’s not fixed because it didn’t have the charger and earphones.then they said they will fix the phone themselves if not they will give me a brand new phone,And mind you they don’t even get back to me and give me feed back I have to be the one that’s always calling and doing follow up, November I went back they told me again it’s still not fixed there is nothing they can do it’s not their fault all we can do is wait because I store is taking long,now it’s Dec I called them again then they told me that I have been approved to get a new phone, and they kept on telling me that I will get my phone any day now, I called again on the 5th if Dec the manger says that they have ran out of the 6s 64g they only have 32g and now if I take the 32g they decrease my installment by 20rnd . Please help because I have been paying for a phone I don’t even use for 3months now,and they not saying anything to me.