Samsung - Defective product

I purchased a Samsung A** model smart phone on **.**.****. At the time of purchase itself the device was overheating while initial setup. But the showroom executives told it's common and no need to worry and told that issue still exist after * days you can bring the device to nearest service centre and it'll get replaced. But after two days there is new problem with the device, we couldn't receive or make a call from the mobile, the call cuts after one ring same for inward calls too. We approached the Samsung service centre at polayathodu kollam Kerala. But they said there is no problem after checking and returned the phone, this all was happening while the device is still having replacement warranty. Then again the same problem occured the next day, then we approached the service centre they said there was no issue, by the time we were really frustrated, the device cost **k, it was gift for my mother and now we are being wandering with this phone like it has been used many years. It was very upsetting for me that they were not able find the fault in the device and also thier casual approach in dealing with customers. My hard earned money was spent in buying this mobile as a gift to my mother and now we are the mercy of the service centre people for getting the defective phone replaced. If anyone can help me in this issue, we greatly appreciate it.