Cellphone contract

I purchased a Sony Xperia M5 in January 2016 and hardly a month old the phone died, when I returned to the store in Port Alfred they only then inform me the phone model has battery issues. It was sent through to Sony in Cape Town. I am still paying for the contract and the cellphone insurance, which means I am entitled to a new handset. It's been 2 months already and I have not been contacted about that. I phoned the Rosebank Mall MTN however they said they would get back to me still nothing. Honestly MTN customer service is poor and I require a new phone within immediate effect. If this is not done I'll file a complaint with the law.