20+ years ago I found the ARE-10 at a hamfest. It was exactly what I needed at my QTH. I was funning simplex and needed a way to keep the radio quiet except when a select group of friends needed me.
Today, the answer is to just use DCS as it's built into every VHF/UHF radio including a $20 UV5R!
This video shows how it was done, old school!
The first DTMF tone is hard coded as "C". There can be up to three more tones. If the the first and last DIP switch are set the same as the middle hardwire connection, only "C" and the hardwire value tone "6" are then needed. Pretty simple stuff.
Once a valid sequence is decoded, a relay will close for about 10 seconds. That closure will connect the internal speaker to the input source so that the RX audio can be heard. That's about enough time to make a call to the station of interest. Along with the closure, a LED will light and latch, indicating that a valid tone sequence was received. After the 10 second interval, the relay opens which silences the radio again. The LED can be reset by the toggle switch on the face of the speaker.
The silencer can be bypassed to permit a QSO by setting the toggle switch to "monitor".