From the National Archives and The Unwritten Record blog

The Signal Corps Female Telephone Operators Unit, known as the “Hello Girls”, was a unit of women who operated telephone switchboards, sworn into the U.S. Army Signal Corps, during World War I. The term, “Hello Girls”, was used for women who first greeted callers with the word “hello”. The corps was formed due to a call by General John J. Pershing in 1917 to improve communications on the Western front. With this call, over 7,000 women applied and 223 women were accepted into the unit. Full story continues at –  https://unwritten-record.blogs.archives.gov/2021/03/30/hello-girls-women-telephone-operators-during-wwi/