MTN-bad customer service,doesnt stick to warranty

My IPhone 6 was with MTN for a month with a display error. The screen went black whilst it was on charge. It is a display error and it is within the 6month warranty that they made me sign. However they sent it back to the store after a month and are refusing to replace the phone and instead are telling me to claim from my insurance - which will probably take another month. I refuse to do this as it is a DISPLAY ERROR problem with the phone. Insurance is not there to fix manufacturing errors, that is what the warranty is for. I have contacted MTN on their facebook page three times, they keep telling me someone will be in contact. However no one has. My phone has now been gone for a month and a half. They need to shape up and replace the phone. I have had enough of this shocking service. DO SOMETHING ABOUT IT!!! Since its on my fathers account I am putting his details and you can contact him because he is livid.