What’s an unspoken rule that absolutely everyone should know, but most people clearly don’t?

People that do this are inviting you to join the call. Announce yourself and ask what the caller is wearing.

The worst are people who do that in the bathroom. Like, hello, I'm trying to jerk off in peace here, stop polluting the air with your banal conversation.

Similarly, watching videos/listening to music on full volume in public without headphones