Cell/mobile phone is just a term based on our frame of reference. For the sake of argument, if it is a time traveler, the device could be transmitting to someone from the future for all we know.
But then I guess if we're entertaining the idea that it's some kind of mystery future technology, maybe we should open our minds to the possibility that it's not transmitting anything to anyone, maybe it has some other purpose that we don't understand yet.
So what we have is footage of someone holding an unknown object near the side of their face. All we really know is that it's some kind of unimaginable technology.
Or maybe it's something completely mundane that they had in their hand.