MTN is a faceless service provider

I renewed my contract with them 14 months ago with an iphone 5s. 9 months later I experienced problems with a non responding screen and the phone was returned for repairs. To my delight the phone was replaced with a new iPhone free of charge. So far so good. Three months later the brand new phone just stopped operating. I was told by an Apple store that the hard drive collapsed and it must be returned via my service provider MTN. MTN advised that they can repair the phone at a cost of R3364. How can I be held liable for repairs to a brand new phone. I received confirmation by the MTN branch,The Grove in Nelspruit, that there was no physical damage to the phone nor any abuse. I requested a written quotation for the repairs from MTN, for insurance purposes with no avail after 5 weeks. When visiting my local MTN store to enquire, I was merely handed a phone and was instructed to hold on. 15 minutes later I was able to talk to a consultant on the line. No solution. Today I again tried to contact the query line. After 20 minutes I gave up.How can a customer be held liable for the repairs to a brand new phone? Why cant they repond to my query?