Go conspiracy theories. This clip has been going around the internet for some time now. Anyway, and now I'm assuming this clip is taken from the actual movie instead of being fake, can you actually see the person carrying a cell phone? No, you cannot. It might look like the person is carrying a cell phone, though that doesn't the person does. People see what they want to see. Either the person holds his/her arm in an awkward position or he/she is a person, managed somehow to get all the way from the future back to 1920 in order to gain his/her debut in a Charlie Chaplin movie, I personally think the first one is a better explanation.
Anyway, before an actual discussion starts on whether travelling through time (though travelling is actually the wrong word, since it implies a movement in space rather than time) is possible or not I think we should leave this to the real scientists. I could do an attempt in explaining it, although I'm sure that many scientists are way better in in doing so then I could do, so I pass. I do study Physics however, but I'm only 16 years old and still have a lot to learn and it's a real pain to see 12-year olds making an attempt to explain. If you really want, then first explain the concept of time, I bet you won't succeed in doing so in the first place.