Sony Experia M5

Two weeks ago I got a Sony Experia M5 from Mtn at Kalahari mall. The phone switched off(battery charged) and would not switch on unless you put it on charge(when put on charge, battery was full). Took it back to the branch where they did not believe me at first so I was sent back with the same phone a day later. After that i had the phone for two days and happened again. Took it back again and they replaced it with a new. Three days later the new one did the same. Now they want to send the phone away because they say nothing is wrong. I did not take out a new contract for a new phone just for it to be sent away for two weeks. This is really unacceptable. I told the people at Mtn that I do not want a Sony anymore but instead they are treating me as if I am the idiot. After speaking to the district manager who was no help at all, I asked him to speak to someone in a higher position and he just said that he did not have a number of someone higher and that I should call 808, which is absolute bull. The district manager should have a number of someone higher in charge. I'm not an idiot. Obviously there was no answer at 808 because call centers are useless. Something needs to be done!