Manufacturing defect phone and bad service

I bought a Sony Xperia M4 Aqua Pearl white from MTN Walmer Park Port Elizabeth cash. The same day I saw there was a blue mark on the screen and took it back within the week you get to take it back. So I got a new Xperia handset, but they said they don't have the white Xperia so they sent me to Mtn Cape Road to pick up a white one that was available. I got the phone but then a week past there were no problems with my phone. That following week the battery only started giving problems, the charging bar started draining and then the same evening the phone didn't charge and the day after it just went flat and hasn't came on ever since.On the 11th of Desember my dad took the phone back to MTN Cape road but they sent him to the MTN walk in centre in Worraker street where they booked it in as a repair to my knowledge now was a mistake according to Mthobeli Ntleli that works at Mtn Cape road and it had to be a warranty assessment. We were told they would try to get the phone back but never did and we called today and no one could help us there. Its been almost 3 weeks and my phone hasnt even been seen by a technician. This is unacceptable and I am not happy with mtn service.<br> Leani vd Merwe