Eli5: what is a secure phone line?

Question

In spy movies they ask, if one is on a secure line? What is that? Can Theoretically any communication channel could be eavesdropped?

In: 20

Answer 1

u/craigworknova covers the ELI5 pretty well, but I have made calls on the STU-III and used them in secure mode. Ours was the version with a key you had to turn to initiate the secure mode.

https://en.wikipedia.org/wiki/STU-III