MNT cell phone contract

On 01.03.2015 I renewed my MTN cell phone contract at Festival Mall. Kempton Park. I askes the salesman what make and model phone is the best currently and he said that the Nokia Asha is very popular. I took this model as pert of my contract. On Sunday 08.03.2015 I took the phone back as it seems that there was a problem with the battery (no battery life). I told the salesperson that the phone's battery is constantly flat although it is charged for a few hours at a time. I was told that this is normal for the battery and that I must take it back home and charge for 24 hours. I took the phone back on 10.03.2015 complaining again that the battery still has a problem to charge and to keep its charge. Eventually the are manager contacted me. He told me to take my phone back to the MTN shop and they will send it to be checked (this left me without a phone for a couple of days). After having to phone back the manager 2 or 3 times did he confirm that the phone was faulty. It was replaced with the same phone - I am still having the same problem, I have a phone that has a flat battery almost the whole day despite charging it constantly. I want MTN to issue me with a different model phone.