The proper response to "hello" is just "hello". I was annoyed at first, but when I saw the phone of one of the kids that kept doing it, I saw his message app had 1283 unread messages. They basically can't keep up with each other, and "hello" is a way of inserting yourself in the queue. That's them calling. You not answering is you not picking up. So ya, ignore it too if you want.
Communication guidelines are necessary and enforcable at a workplace, but this is also no big deal. Being annoyed by it is a problem, and really is your problem until you make it a problem for the person doing it. And it becomes a power issue. So without the guideline and without it being enforced, they may judge you for judging them or be offended by you thinking you can change their behavior.
With that said, communication is a skill that does seem to be lacking. A lot of so called "teams" will have information going around in circles in a game of telephone. Then there are those who can't seem to grasp what is important, and keep deflecting to sentiment or respect issues.
Bill Maher jokes about how millineals are emotional hemophiliacs, but regardless of the characterization the problem is real. You'll offend them and they'll cancel you without telling you. You become someone they talk about behind your back. And you'll cancel them for that. But cancelling each other out doesn't work if they're a client or a coworker that isn't going away. If they work for you I guess you can fire them, and if you work for them, I suppose you can quit. The only other option is to learn their language.
Ergo, communication remains a skill.