I work outside random concrete buildings and need to communicate to people inside. Radios don’t work. cell phones are spotty. So I want to run a loudspeaker inside on a 2 or 4 wire cable and be able to talk to them like a PA system but I want them to be able to push a button and talk back through the loudspeaker preferably or a mic. there may be something I could buy already built. but can’t find one. I’m not sure what this system is called either. I’m looking for components and a circuit diagram I could use. Any help or ideas would be appreciated