Sort of unfortunate, because there's really no good translation for "hello" into Japanese - you'd say こんにちわ in the morning, in the afternoon こんばんわ and もしもし when answering the phone...
you'd say ohayogozaimasu in the morning, konnichiwa in the afternoon and konbanwa in the evening.
I don't know how to type japanese on my phone, but the first is literally "early" surrounded by honorifics. The characters for konnichiwa means "this/now", "day" and the "wa" at the end is an article making the previous phrase the subject of the sentence. Same with konbanwa, but for evening instead of day.
no idea on the etymology of moshimoshi for answering the phone, though.
According to an article I read, moshimoshi came from the telephone operators saying 申し申し (moushi moushi) to signify "I'm going to start speaking now". On a tangential note, 申し used to be the phrase when calling out to someone to ask something (similar to English "Excuse me").