bad service from MTN

On 2nd August 2015 me and my sister did an upgrade on our MTN cellphones from an MTN shop in Mall@Carnival. A while went by and my sisters phone got ****** and when she decided to claim it from the insurance the let her now that after the upgrade we did. The new phones is not on the insurance and that they will only pay out R2500 for a Galaxy S6. When we started asking the shop and MTN direct and head office for a copy of our contracts. It was made clear that MTN does not have contacts for our phones. Why are we then paying if there is no contract? My phone Galaxy S6 edge fell and somebody told me that I can get the screen fixed for free I just need to take a copy of my contract to the store and the send in my phone for free. I requested a copy of my contract to do so. And once again MTN head office informed me that there is no contract. I am now sitting with my hand tide up and do not know what to do. I have gotten a total of 6 reference numbers out of MTN and nobody can help.