Not replacing broken phone

I upgraded a phone at MTN about 3 weeks. I got a sony experia m4 but the camera did not focus. It was a hell of a fight to get a other phone.Then eventually got a other phone but it is also faulty, it goes off by itself and then it is a hell of a job to get it back on again. I showed the salesman(Gerrit) and he said that it is the 3 phone in his store with a problem. He will tell his boss and come back to me. I phoned again after a week, but still nothing yet and then after another week still nothing. Today I phoned him and he said he is still waiting he had send 4 e-mail to the warehouse nothing yet. I said please give me a phone number of any manager he said he cant. I said but you cant help me I need someone who can help me, I have got a broken phone and MTN does not want to replace it. I then wanted to cancel all 4 my contracts at MTN and he told me I cant. I don t know what to do. I just want this phone replaced or my contact cancelled. Can you please help. It cant take this long!!!