Iphone5 call pick up issues

Hi,

The last few weeks (I think) i am having issues picking up a call.
My phone rings, I pick up, I hear the other party, they don't hear me.

I call back and all works fine.

So far this is what I have done.

1) Power down power back up. - No luck.
2) Called apple. They told me to do a down load with no issues noticed.
3) They made me do a full restore and I did not put my back up back.

Still the same issue.

Anyone an idea. All help is welcome!!!

Thanks