Samsung Galaxy I5503 — Samsung Galaxy I5503 defective piece sold

Address:

Bardhaman, West Bengal

Hello Sir,

I bought a Samsung Galaxy I5503 worth 8500/- from the shop named "Samsung Mobile" in Benachity, Durgapur, West Bengal in the first week of November,2011.
Since I bought the mobile, there has been several problem like frequent hangs, huge battery drain, switching off suddenly, 3G not working properly and several others. I went to the shop but they were not ready to listen to my problem and told that I should go to the service center.
I went to the service center and they did some preliminary checks and told that the problem is solved. After that, again the same problems happened and I went to the service center again. They again did something and returned me the mobile. Till now, I have bought the mobile before two months, and in these two months, I have visited the service center for three times.
Why it should be like that? I am pretty sure that the mobile that was sold to me was a defective piece and I think it should be replaced with a new one. Going to the service center once in a while is ok but not three times in two months and not sure how many times in future I will need to go.
Sir, please let me know whom should I contact for this problem and how should I get my defective mobile phone replaced.

Regards,
Animesh