I think "hello" was regarded as somewhat informal, even crass, before it was used extensively in connection with the telephone. "Hello," coming from a hunting call, was sort of the equivalent of "hey you!" It was recommended for use with the early telephones because it was appropriate for calling out to get someone's attention -- which was necessary given the limitations of early telephony. Plus, it was better than the alternative "ahoy hoy!"
As for movies in general, I think every period movie reflects the time in which it was produced. It's inconceivable that a movie like Gone With the Wind
could be made today, given it's rather cavalier approach to slavery and gender relations. That's a movie that says quite a bit about the 1930s as well as the 1860s. Today, Rhett would be charged with spousal abuse after he sweeps up Scarlett in his arms and takes her up that grand staircase. And let's not even talk about The Birth of a Nation
Oddly, I've found that the movies that pay the most attention to getting the period details right are often seriously flawed in other ways. Eight Men Out
was particularly scrupulous about getting the details of early 20th century baseball right, but it was, in the end, a pretty dull film. Changeling
was meticulous in recreating 1920s Los Angeles, but it was generally panned by film critics (although I thought it was pretty good). And the makers of Gettysburg
were, I think, intent on getting every detail correct, down to the buttons and belt buckles, but despite that it was a terrible movie.