I don't see how "hello?" helps with that. If anything, it makes things worse if everybody does that, because now half of those chats from 30+ different people are that, drowning out the useful messages.
In the described scenario (chats falling off the front page) it doesn't help either party.
In the case where you want an immediate discussion, don't trust the status indicator, and there are multiple different people you could contact, a quick confirmation that the selected party is online and available is not unreasonable.