Poor service from MTN Service provider

I upgraded my contract with MTN in December 2014 to a LG G3, I used the phone for about 2 months and noticed hairline cracks on the top and bottom microphone of the device and also the phone should overheat while charging. Before handing in the device to MTN, I researched the problem on the internet and the feedback I got was that it was a LG factory defect and LG was replacing the devices. Upon handing my device to the MTN store I upgraded with I informed the sales person about the research I did to why the device had cracks and he also researched it before sending it to the service and repair centre at MTN. In response from MTN service and repair centre they said it was physical damage to the device and it failed warranty, therefore they repaired the device and sent it back to me. I used the device for a week and it started to trouble by switching off and the screen going blank, Again I went to MTN to hand in the device a 2nd time. After about 2 weeks I heard that I'm getting a refurbished device as my device could not be used again. I was not happy receiving a refurbished device as I am paying for a brand new one. MTN should of researched the problem first. Very unsatisfied.