Refusing to take back a new but damaged phone

I bought a new phone two weeks back, now it is overheating, wanted to return it to get a new one, now they are refusing to grant me a new one and are telling me that it has to be taken in for repair. Now, I'm supposed to be without a phone for 7 days or more. The store is telling me about the phone's 7 day policy, but I bought the phone from their store and not from hisense directly and any product is supposed to be returned within 30 days or so. So I'm punished for returning it after 7 days, like I was going to guess that after installing whatsapp the phone would overheat.