Default Problem with Voicecalls

Hi There!

I have a Blue GS3, in NZ on Vodafone NZ Network and a reasonable number of times when I am on a call the other end says they can no longer hear me. If I call back they can hear again but the call will be lost on one end (Always the other end so they can't hear me).

I know the antenna is on the bottom (Who thought that was a good idea) when I commonly have my hand to old the phone, but wouldn't that mean sound in both ends would be bad?
Galaxy SIII