German 'hallo' and English 'hello' share Proto-Germanic *halōną (to call) — both repurposed as telephone greetings in the 1880s.
German greeting used when meeting someone or answering the telephone; cognate of English 'hello.'
From Old High German 'halâ' or 'holâ,' the imperative of 'holôn' (to fetch), originally a shout to hail a ferryman. Through Middle High German 'holâ,' it survived as an attention-getting exclamation. Both German 'hallo' and English 'hello' adopted their greeting function in the 1880s with the arrival of the telephone. English 'hello' shares the same Germanic root — from Middle English 'holla,' from Old French 'holà,' itself borrowed from Germanic. Key
Neither 'hallo' nor 'hello' was used as a greeting before the telephone. Thomas Edison proposed 'hello' for answering the phone in 1877; Alexander Graham Bell preferred 'ahoy.' Edison won the battle in English, and German adopted 'hallo' — previously just an exclamation — as its telephone greeting in parallel, transforming both words from shouts into salutations.