system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: In movies, why do secret agents always request to go to a secure line when on the phone. Why not be on a secure line always?
gpt
I think the trope was, originally, you call someone and they don't know if you're calling from a secure line or not; or you don't know if the random phone number someone gave you is to a secure line or not. This was with 1960s  technology, so it's not that the line was encrypted/scrambled, it's more about stuff like "are you calling from the house phone that the KGB probably has tapped, or are you calling from a randomly selected payphone in another city".

Then the trope got carried forward to modern movies even though the notion of a secure line means something different now.