Well, this is pretty crazy: someone found what appears to be someone using a cellphone in the background of Charlie Chaplin's 1928 film The Circus. Go ahead and take a look for yourself:
The footage in question starts at 2:43 in the below video. What do you think? It sure looks like someone holding a phone up to their ear and talking into it, doesn't it? What could it be way back then? Other than a time traveler, of course. Which I will assume it is unless proven otherwise.