Hardwired phones? If I were to run a wire between buildings, I would use a single fiber optic strand. In fact, If I were doing any type of on-site communications, I would consider a cave radio system using electromagnetic fields in the audio frequency range. If one wanted to communicate a couple miles without using RF, a few laser pointers modulated with audio (easy) would do the trick (Starlink sats also talk to each other with lasers fyi). You wouldnt even need lasers with fiber optic though, LED bulbs from a flashlight or christmas tree would work, even for the sensor. LED lights of the same color can be used to pick up the signal from the active one, so there's no need for fancy photo transistors.
Realistically though, if you are close enough to run any wire, you can use RF with little risk of detection as the power levels would be so low (sub mW if you know what you are doing), nobody would find it. The day the world goes "radio dark" is the day a satellite or drone pass-over will be the first stage in finding human activity. Unless you know where the underwater servers are sitting 200 feet below the ocean surface, you won't be erasing google satellite photos any time soon. There will be so much RF come that day that you'll have bigger problems to consider.
