Very poor, shocking business

I have a contract with MTN that I took online via Istore about 2 months later iphone started freezing and getting very hot so I called MTN and I was told to book it to the nearest MTN. I did just that and now my phone is been gone since 05 June, I can't get anyone to account for this. I'm currently paying for a phone I don't have. It's not even my fault as the phone had a manufacture fault but I'm suffering the most. You need to sort the out or I will take my money to service provides who deserves it.