is used to call for someone’s attention. Although it is often used on the phone, the use is not limited to phone calls. is a repetition of , which is also used to call for an attention. is a variation of (), which was used in the same way in old time. definitely predates telephones, and I guess that both and for asking for an attention predate telephones, too.
The use of in this meaning is archaic. Both and in this meaning sound a little old-fashioned to me except for the use of on the phone.
Some people write on the web that is a contraction of (“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 other than on the phone may be children’s song . The lyric starts with .