Please help me

I upgraded my cell phone at the end of April and on 21 May it died. I got a Sony Xperia Z3 compact. I took it back to cellucity on 22 May and after examining it and seeing it was still in perfect condition they said they would send it to Cape Town and if they couldn't fix it they would replace it. After enquiring about it they told me they had sent it to Joburg to be fixed. I eventually contacted the service manager in Cape Town to enquire what was taking so long. On the 9 June they told me they had replaced my phone and I could pick it up. The back of the phone they gave me was scratched and the phone kept getting hot. I contacted the sales manager again by email and he told me to take it back to N1 city with the box and he would see what he could do. I get an email today to say they refuse to give me a new phone, they will order a new back cover for the phone but it will take a month. I need a phone. I am a pastor in the Salvation army and I need to be in contact with my congregation.