Faulty Battery

I bought a phone last year in July 2014 and already the phone is off due to battery problems. I went to two MTN outlets (Checkers and Thabo Mbeki street)to query the problem I have on the phone. I was told the phone didn't belong to them as the code on the proof of purchase was not theirs. I was surprised because when I was called to the the phone , the caller mentioned that they were representing MTN. I then went to MTN repairs in Polokwane with a referral, my phone was taken in but nothing was done on it. The only communication I received was an orange note on the battery written faulty accessory. I don't know if wether I have to buy a new battery myself while the phone is just 9 months in my hands. I am not happy with the device because to me it is a factory fault. This is the 1st time this kind of thing ever happened to me. What is the way forward since this phone is still under WARRANTY? Regards