I have been using an Autocom system for several years, and maybe I'm just spoiled, but the Autocom, as well as most telephone systems plays back the microphone into the headphones so that the user can knows 1. the volume at which he is speaking, and 2. that the device is actually working. Past research showed that without such feedback into a telephone headset that people would generally talk much more loudly than necessary. I would like to expand that idea slightly though, and have the feedback come directly from the receiving device. That way, when you hear your own voice in your headphones, you know that it has been received by the other headset and returned to you.
This could be done in software, or possibly with the addition of an electronic feedback from the mic to the speakers.