Does the term "もしもし" (moshimoshi) predate the telephone? Does it have any use besides answering the phone? Where does it come from, is it just a reduplication of "もし" (moshi) "if", and if so how does that work?
もしもし is used to call for someone’s attention. Although it is often used on the phone, the use is not limited to phone calls. もしもし is a repetition of もし, which is also used to call for an attention. もし is a variation of 申し (もうし), which was used in the same way in old time. 申し definitely predates telephones, and I guess that both もし and もしもし for asking for an attention predate telephones, too.
The use of 申し in this meaning is archaic. Both もし and もしもし in this meaning sound a little old-fashioned to me except for the use of もしもし on the phone.
Some people write on the web that もしもし is a contraction of 申し上げます、申し上げます (“I will speak, I will speak” in a humble and polite form), but I do not know how reliable this claim is.
By the way, one of the most widely known uses of もしもし other than on the phone may be children’s song うさぎとかめ. The lyric starts with もしもし かめよ かめさんよ.
もしもし is from 申し（もうし） being double and shortened, and at Edo-era people use only もうし without repetition.
申す(もうす) is same meaning with 言う(いう)/話す(はなす), but we use as polite-from nowaday.