Time Traveler in 1920 Charlie Chaplin Footage?

Filmmaker George Clarke has spotted something unusual in some "behind the scenes" footage of  Charlie Chaplin's 1920 film The Circus. In the brief clip, a woman walks into frame talking while holding a small, thin device up to her ear. If the film were made today, we would assume she was talking on a cell phone, but this is 1920. Clarke thinks that the woman is a time traveler, but if the device is a cell phone, to whom would she be talking?