Dispute when it comes to repairs

Last week Saturday I signed for a new contract. I received a brand new Sony Ericsson M4 within three days the battery was getting hot and the screen would not switch on. I took it back and they sent it in for repairs. I got a call today telling me that my phone is not faulty but three days after I receive the phone the screen will not switch on. They are telling that there is no fault but the screen will not switch on after three days and they just have to take back my phone. I believe that even though they are saying there is not fault I still believe that as a consumer I cannot take the phone as I am concerned that the phone will land up just giving problem after problem. The store manager is unhelpful and was not willing to deal with me directly which is poor service she will rather use as other people to deal with the problem. Overall it is very disappointing to deal with Telkom mobile as they cannot see that to take this phone back when they were not even to say what the problem could have been.