Why would people say "hello" whenever they pick up the phone? Since "hello" and "hi" are interchangeable, would it be okay to say "hi" instead?

Native speaker’s answer
Rebecca
Yes, it is fine to use "hi" instead of "hello". I would say it's not as common, but it's perfectly acceptable. "Hello?" has become a sort of question asking who is there as soon as you pick up the phone, and then you use "hi" when you know who it is. Or, if you are the one calling, you can introduce yourself by saying "hi". Ex: Hello? Who is this? - Oh, Henry! Hi, how are you? Ex: Hi, it's me, Finn! I would like to ask you about the dinosaur in your garden?