A pleasantly discursive Cardus post by Nate Barksdale examines the history of “hello” as a telephone greeting:
Hello streamed into the gap created by an unprecedented social scenario, gaining popularity and, little by little, respectability. By the 1920s, Emily Post had given up on banning hello from her version of proper speech and simply tried to tame its former brashness: “On very informal occasions, it is the present fashion to greet an intimate friend with ‘Hello!’ This seemingly vulgar salutation is made acceptable by the tone in which it is said. To shout ‘Hullow!’ is vulgar, but ‘Hello, Mary’ or ‘How ‘do John,’ each spoken in an ordinary tone of voice, sound much the same. But remember that the ‘Hello’ is spoken, not called out, and never used except between intimate friends who call each other by the first name.”
… The fact that the message did not depend on the word itself was probably as key a factor as the device’s American pedigree in the internationalization of the telephone hello. This was especially [true] for languages that have an active distinction between the formal and informal you. In Bulgarian, say, the formal greeting is zdravejte, while the informal is a simple zdravej. The phone rings in Sofia: what do you do? Is the caller a friend or a stranger, an official, a salesman, a wrong number? Will it be zdravej or zdravejte? I know, alo!