Mtn worst service received and still no answer???

I am not one to complain about service and products but at this point I am highly disgusted in the service received from mtn and Sony south Africa. I upgraded about 10weeks ago received a Sony xperia Z5 and within 10days realised that the battery Sony rates last 2days only last 6hours! Took the phone back to mtn in Newmarket Mall were the 2 managers looked at the phone and said they think the battery has swollen. Needles to say no one came back to me I have to go to the store multiple times to find out what is going on. After a lot of fights I went to head office still received no help! After using social media I got instant reply asking for I'd and cell phone numbers I sent it straight away on 21st of December now once again no reply this is really not on!!!! How can I pay for a phone that does not work