Defective within 6m Warranty should be replaced!!!

Defective Samsung A7 device was taken to MTN Nandi Drive Repair Center [River-Horse valley] being under warranty. MTN Technicians advised that the warranty was dropped due to phone being rooted resulting from downloading unauthorized software. Which I refuted! After a month back-and-forth, Samsung requested I fetch the device from MTN and bring it to their Samsung Repair Center. Samsung have repaired the device under warranty, and gave me written proof explicitly stating that the nature of the defect was a factory fault and the device was not rooted or tampered with in anyway. I have submitted this proof together with the device requesting a new device since according to my contract the device should be replaced being PROVEN defective in less than 6 months. Its simple, I should get a new device, its going on 2 months (Initially booked on 05/10/2015) without my device or a solution. Constantly chasing up with MTN with no attention, I feel this matter is not being taken seriously!!! PLEASE HELP