The Wildtalk designed and built wired full-duplex beltpack.
Full-Duplex systems are available in a number of wireless (and wired) technologies which are outlined here.
Wired Full Duplex
A wired solution can be the lowest cost solution ideal for users that do not need high levels of mobility. We have developed a simple and very easy to use wired system. It is compatible with virtually any headset from any manufacturer. It is plug and play so is exceptionally easy to install. It has an internal speech processor allowing it to be used in both high noise and quiet environments.
In our experience this is the most reliable and longest range wireless solution. We have achieved over 2km. It supports dozens of headsets in one area, suffers no interference and provides the best speech quality. DECT products use protected radio spectrum and auto-negotiate their use of wireless frequencies and so are interference-free.
This is the lowest cost solution because Bluetooth uses a part of the radio spectrum called 'International Science and Medical' (ISM) which is available to use in nearly all countries worldwide without regulation and so Bluetooth products and can be massed produced. However the ISM spectrum is also used by Microwave ovens, Wifi and many other wireless devices so occasionally in some (urban) locations Bluetooth can perform poorly with crackling and drop-outs because of spectrum competition from these other devices.
This system has the advantage in that it can use existing Wifi networks so a low cost network can be setup quickly extending range easily using standard wireless network components.
800Mhz ISM Wireless
There are a few products using this band of frequencies. There is little competition from other technologies in this band at present so it generally performs well.