saying "hello" when somebody asks something obvious

This is another example of how quickly something can catch on. I cannot pinpoint the first usage, but it was spread by television and movies to the point of satire.

Saying Hello?!? in this manner is a reference to speaking to someone on the telephone. Presumably when someone says this, they are asking if there has been a disconnection between the two people having a conversation. In other words, they believe whatever they are saying not to have had the appropriate impact or response on the other party. And, had the conversation been via telephone, they would suspect a disconnect to be the only reason for it.

The usage as an interjection when describing other people's lack of understanding is just an extension of this.

So, I told the guy, Why the hell would I want to pay you more for these tires? Costco sells them for 50% off. Hello?!?

As a note on its usage: there are many (myself included) who find it very obnoxious.

There is another interjection Hello!!!!. This is used as a means of expressing surprise. And it's usually pronounced hell-oh!!! with accent on the second syllable. This meaning is just a usage of hello to "greet" an unexpected idea or circumstance.

Hello is simply making sure the person is "alive" or still there.

One reason to say it is if the person has drifted off - where hello means "wake up".

Another would be is if someone isn't getting something - where hello means "duh".

Usage: Hello?! Hello?! Is anyone home. Don't tell me you only ordered three tires for your car.

This is a dated phrase although still used. Very popular representation of it was in the Back to the Future movies, when Marty goes back in time.

I believe this form of sarcasm is not restricted to English speakers.

Therefore, it is not so much as what the phrase or word means in English but what it means in a social and relationship context. Rather, what do people mean to tell you, in whatever language you are conversing.

Henry: Are we going to school today?

Jessica, cutely gesturing with knuckles knocking on air next to Henry's forehead:
Hello, knock, knock, any body in there? Today is Saturday, dumbo!

Another situation...

Henry: Are we studying together tonight?

Jessica, shaking her head and up in arms in disbelief, after Henry had broken off with her last night, looking straight at his eyes:
Hello?

We could simply explain that the person who is telling you "Hello?" is questioning

• if you are paying attention to reality of the present situation.
• if you have any common sense still intact
• if you had any intelligence
• if you are still alive in there
• if you are still human
• wtf were you thinking?

But explaining it in words what the verbal gesture means would be inadequate and not do it justice. The possibilities are many, but the sarcasm is obvious, in any language.