I cant hear voice when I am calling or receiving call. iPhone 4

I saw some voice problem messages in the forum but I have different issue. I am an old İphone 3GS user. I have bought a iPhone 4 in Apple Store in Xanadu (Madrid/Spain)when I am in spain for a formal meeting, in Dec.17.2010. After my bussines trip, I have turned back my home and I have activated my new iPhone 4 by iTunes 10. But there is a problem now. When I am calling someone or receiving a call, I cant hear any voice. But they can hear me. This is very interesting issue for me. I have never got any problems like this with my iPhone 3GS. Help me please.
iOS 4 iPhone4