No audio in calls, but games and other apps are fine?

I recently bought myself an iphone 4s, its running the latest firmware.

When i make and recieve calls.. I cant hear the person on the other line and they cant hear me... But when i use apps such as skype, the mic and speaker works fine, in games the sound works fine, when using siri the mic works fine.

Everything works as it should, its just when making and recieving calls.. any ideas? anyone heard of this happening before or experienced it?

Ive tried restoring the phone and re installing the firmware and setting up as a new phone but ive still had no luck.