If you’ve come here after seeing the explanation in the Teen Wolf TV series where one character explains TO the Japanese character (who somehow does not already know, despite being Japanese), that ‘moshi‘ means ‘hello‘ and that by saying it twice you can be sure that whoever is on the other is not a were-creature of some sort, because such creatures have difficulty with long sentences!!!
You can feel safe in the knowledge that Teen Wolf is completely and utterly wrong. In fact, you’ve never heard such garbage in your life.
Moshi does not mean Hello. Lots of other words in Japanese mean hello, the closest matches being O-hayo or Konnichi-wa. Moshi means ‘calling’ – and the, very simple, reason Japanese say ‘moshi moshi’ is because it dates back to the early days of telephone communications, when there were no dial and ringing tones, and people generally said ‘calling… calling…‘ to announce they were on the line until both sides could hear each other.
So ‘moshi moshi’ is just ‘calling… calling…‘ in Japanese.