The Contran shown in post 1465 was conceived by an airline Capt. in the US, in response to the KLM/PanAM collision on Tenerife in 1977. He patented the device, which needs be only some simple added circuitry in a VHF Comm, not a separate LRU, Line Replaceable Unit.
Even VHF Comm design engineers, while not the brightest at most manufacturers, could see the drawbacks to the idea. In busy airspace, pilots would soon get in the habit of keying the mic before the present transmission is finished, in order to be next to talk. As soon as two or more do that at the same time, the system is fragged.
There may have been other factors that doomed it, such as NIH, Not Invented Here.
GB