This customer wanted wireless intercoms to place on the outside of bulletproof glass so they can talk to people on the other side. We recommended the WireFree Wireless Intercoms for this application.
The WireFree system has a VOX mode that enables you to press the Talk button on the inside intercom and then the person on the other side of the bulletproof glass can speak without pressing a button. They just simply speak and you hear them (there is a two second delay before the microphone is activated). You can interrupt them at any time by pressing your Talk button so you remain in control of the conversation. They can speak for up to 15 seconds before the microphone cuts off again. You can simply press the Talk button again for another 15 seconds (both units must also be set in Conference mode as well).
There is a Monitor mode on the WireFree system that you could activate so the microphone on the listening end is always activated. You will then hear everything that occurs on the monitored intercom. If you want to talk, you have to press the Talk button on your intercom. Both intercoms should be powered by AC to DC power supplies since batteries will be quickly drained.