MTN change warranty policies without informing you

I have had my iPhone 5c for 16 months. Purchased on contract with MTN. The screen on the phone has faded to black, having never been dropped or undergone any damage to the unit. The phone no longer works since the screen faded to black. Apple tech support suggested it is a manufacturing defect. My MTN CONTRACT WARRANTY forms clearly state that my unit device carries a 24 month warranty, which covers repairs free of charge. When I have been to the MTN stores at Stoneridge and Eastgate (where the contract was registered) they told me their iPhones only carry a 12 month warranty. NOWHERE in my documents signed from MTN in our contract agreement does it mention that. This is absolutely ridiculous! How dare MTN change their policy without informing me, I expect them to take my device and repair it free of charge because it IS still under its 24 month warranty!