Sometimes no sound when i call someone with iPhone 4s...

Hey,

I just get my iPhone yesterday and have a "big" problem.

When i call someone there is sometimes no sund and i cant hear the person who i am calling. But i see the minutes on my iphone that i am calling someone. But this is onl sometimes and only when i call someone. So if there is no sound and i plug m headset in my iphone ad plug it ou then i have sound in the speaker. It seems to be an software problem. So i restored the iphone 4s to an new iphone and the same mistake!

some solutions?