Imagine the beginnings of phone sex, right after the invention of the phone and until only a decade or so ago, most homes only had one phone line and a few extensions, so having phone sex on the family line was a bit of a risk unless you were ok with someone possibly picking up the phone mid conversation. If you lived alone or were in a hotel room or something it would be private, but in a home with others, not so much.
Way back when things weren’t even automated, a live switchboard operator had to manually connect the calls and they could have listened in to any of them and likely did to any entertaining ones. Switchboard operators in a small town likely worked alone and had privacy, imagine them masturbating if they happened in on such a gem of a conversation! It likely would have happened and they just kept quiet about it. There’s a difference between that type of listening in ad if you are in a hotel listening to the couple next door fucking. If they are being in the slightest bit noisy, they know and just don’t care anyone can hear them. They likely are getting off on it.
I was in a hotel a few months ago and a couple was going at it and I put my ear tot he wall to hear even better and ya know what, I masturbated as I listened to them. It was hot and I could hear every thrust and I was rubbing my pussy at the same pace and came right after they did, it was very sexy and they did it a couple times that night and I masturbated every time right along with them. Listening in can be hot.