Staffs lack of knowledge/unwillingness to help

So my iPhone 6+purchased from the iStore - Eastgate on an MTN contract early November last year had a few issues - overheating & it tends to freeze on wifi networks at times. I took the device through to the MTN repair centre as instructed by an iStore assistant in the Sandton store. According to him the devices warranty lies with MTN and they need to sort this out. MTN took the device in for an assessment and had confirmed the issues being experienced. I received approval from MTN in the form of a reference number on the documents handed to me when the device was booked in. I was instructed to get a replacement device from the iStore Eastgate as this is the store my device was purchased from. Before going to the store I made a call to check on their stock availability. This is where the incompetencies arise. Upon my request being logged I received a call from the Eastgate store basically saying MTN is responsible for handing a new device to me. MTN say my device was purchased from the iStore and nothing can be issued from their stores?Now 3 days later & without a phone..NOBODY (MTN/iStore) knows *** to do or how to resolve this? It's an embarrassment to the Apple brand & MTN alike