Phone not working and management does not call us

I took out a contract on January 2016 sony xperia m4 then the phone freezes and overheated and switched off by it self , i took it back in they send it to repairs department and gave me a new sony xperia m5 but i still get the problem then i phoned the kempton park store and told them and ask if we can change the contract they said i must speak to isak from mtn he did not call be back since 12 april 2016 and i tried to call him a few times and i whatsapped him but he respond saying that he will phone me on the 12 april 2016 but he didnt , all i want is a phone that works my phone switches off and freezers and overheates please help me as i am trying to get to the corrected person to help me but no one wants to help i even called the 808 number they said go back to the store i just want a phone that works