Sony phone issues

My Sony Experia Z switched off and couldn't switch back on, I took it back to MTN Gandhi Square where I bought it and two weeks later it came back with a damaged battery. I took it back immediately and when it came back it couldn't read the sim-card. I took it back again and it came back with the same issue. Now for the 4th time I have taken it back to MTN and I am informed I have to pay an amount of R1300.00 for something that I had nothing to do with. To add to my frustrations of not having a phone I'm being charged for something I had nothing to do with. It has now been going on since December. Why am I still suffering when monthly I get charged and billed for a phone I don't have.