Received defective phone when bought new

On Monday 17 Aug'15, as part of our upgrade, we took a new Samsung Galaxy S4 mini phone, after all data was transferred, on 1st time using the phone, the screen started freezing. on return to the branch they confirmed that the phone is defective. They 1st tried a software upgrade, no improvement, the phone was then send to Samsung who returned the phone saying there's nothing wrong, the phone was then again send to another agency, still no joy.<br> It's now been exactly a month, and after many calls to the help center / customer care line, being shunt around from one person to the other, from one section to another, still no joy.<br> The Branch does not want to replace the phone in the interim, nor do we get any response from them regarding the status of the phone, and I'm not allowed to cancel the contract.