I have bought an iphone 4s.. My music plays, games plays audio, my audio and mic works fine on Skype, my siri works fine.. BUT when I call someone or someone calls me I cant hear them or they cant hear me?
Have you tried resetting network settings? Settings>General>reset>Reset Network Settings You will need to add your wifi passcode back in afterward. Not sure if it will work, but worth a try.
A friend had the same problem, them hearing him only, on his 4 and it turned out to be the second mic was bad. He was still under AppleCare and had his phone replaced.