We have noise reduction headsets, so obviously to get the full effect the headset has to be worn over both ears, and the intercom has to be used. It's a right pain in the butt flying with someone how insists on not wearing a headset. Depending on my mood at the time I might take one ear 'off', or make them repeat themselves constantly until they get the message.
It's very noticeable that colleagues who fly with their headsets off in the cruise miss far more radio calls.
Just at a tangent, can anyone explain why PA's (in our 737-500s at least) come across clearer through the handmic than the headset?