[sandworm101]

And a reminder of how unsecure and unencrypted these communications channels were. That operator not only could listen in, but was certainly instructed to listen in on all such priority calls in order to police them. No encryption. No channel hopping. Anyone with a handset and a pair of alligator clips could have literally clipped into the line at any point. Anyone in the know would also be able to monitor a trunk line looking for that priority signal. So it would probably have been more secure to hide amongst the masses by not pushing the priority buttons.

[yencabulator]

Much like today's TCP/IP world, the encryption was layered on top of the phone call. The military used codebooks, code words, and authorization codes.