Why two-way radios make you say "over" — and why cell phones never make you say it at all.
Every radio link has to answer the same question: how many directions can information flow, and at what point in time? The answer sorts every system — broadcast radio, walkie-talkies, cell phones — into one of three buckets: simplex, half-duplex, or full-duplex. The difference isn't a minor technical footnote. It's the entire reason a police dispatcher says "copy, over" while a phone call just flows like a normal conversation.