No service from this 'service provider'

I first bought an iPhone 6 in March this year shortly after which I started experiencing issues with the touch pad and I took it back to Centurion and they exchanged it. After a few weeks I had issues with the speaker and went back to Centurion where they gave me a new phone again. I experienced issues with the speaker again and I took it to Centurion where and they 'fixed' it but I also spoke to the manager to request being moved onto a different package(phone) because it's ridiculous that I was on the 3rd iPhone in under 6 months. Clearly the iPhone 6 is faulty hence they keep giving me a new one but I can't keep changing phones and I'm actually willing to pay more to move onto an iPhone 8. The manager in Centurion wasn't very helpful and he said I need to wait till i'm due for an upgrade which feels like a lifetime away given all the problems I've had with my current device. The speaker is still faulty and I've moved to Randburg so I took it to Cresta where they advised that they needed to send it back to Centurion as it was still within 3 months of the previous 'repair'. Yesterday I got an sms that my phone was ready and when I picked it up the consultant(Johanna) said they'd returned it and not repaired the comment was 'store used wrong account number'. I'm at wits end right now!