Bad service and no help

On the 7th of Jan 2016, I bought a Sony Xperia M5 and when returned home, the phone did not work and it refused to switch on after I had charged it for over 5hrs. I took it back the next day and after waiting the whole day and having to go back and forth, I got a new phone. A few days later, my phone suddenly switches off and it continued for two days, I called in yesterday 16th Jan and I was told to wait and see if it will continue. It continued and I went to MTN today, 17th Jan and the service was really bad. I was told to wait for the person who sold me the phone and I waited for 25mins only to be told the Sony technicians are not in and I would be called tomorrow. Firstly I was misled by coming to MTN and I wasted my time going there and my issue is not resolved. I'm truly not happy with the service and I was assured I would get a new phone tomorrow and I hope Mtn delivers and I don't want to be pushed around again.