I have Sony Ericsson w810i....?

I got Sony Ericsson w810i mobile set. Its all good, but only i found the network strength is worse. It always says "Unreachable" when I dial to myself, but in mobile set, the tower is full. About 80% of my call is missing because of network problem. I went to vendor, and he said its a problem with network, the service provider. And when I contacted the service provider, they said you have defect on your phone. What could be the reason???