Bad service

I had renewed my MTN contract 14 Nov 2015 and took a Sony M5. After a week the phone went faulty.It switches off and only switches on when u charge. I took the phone back to the store on the 21 nov 16 on the 7th day.Instead of replacing the phone thats OBF the phone was put on charge and switched on again despite seeing there was a problem we had to take the phone back and continuosly experienced the same problem.Finally on the 6 Feb 16 we dropped the phone off for repairs and was assisted by the manager Victor who assured us the phone is under warranty and will be assessed and replaced. The phone was checked for defects and was in mint condition and we have that on record. We unable to get hold off mtn until today the 22 Feb 16. they say that the phone has damages and something about headphones missing which is total **** as we gave them the phone in mint condition with headphones still sealed and never used and we have that in writing. The service from the start was pathetic and now we being cheated over again! How can MTN treat customers this way. I want my phone replaced or my contract cancelled immediately!