why my brand new iphone 4s while making a phone call or outgoing calls, the receipient can't hear me at all except i use loudspeaker?

I just bought iphone 4s. but i just notice that if i made a phone call another people can't hear what i said to them except i turn the loudspeaker on.

it is really irritating since sometimes i need to yell so the one i caller can hear me out.

i tried to ask people and even the one who sold the phone said to that the phone has something wrong with the hardware.

what can i do??

i tried to restore many times.

it is not working.