A cellular phone in the 1920'swhere are the transmitters and receivers?
Either that or it's a beam me up Scotty Star Trek communicator
Or much more likely she's got a tin can to her ear and a long thin bit of tight string between her and her friend behind her, camera right, off stage
Or - Or, much more likely, the movie is a clever fake, dead easy (relatively speaking) to do this convincingly nowadays, show me some 90 year old original film stock that has these images on it!!