Without the ability to network there is a limited distance by which they can communicate. Fundamentally, there is a transmitter and receiver. There is only a certain distance by which you can transmit based on the antenna, power, and signal type. In an area packed with building and obstructions the typical distance is not far. A cell phone usually implies it can talk over the telephone network (i.e. reach any other device connected to that network by passing along the signal), a radio essentially allows data transfer on a given frequency, or group of frequencies. Both are subject to noise and dB loss on the EM wave created by the transmitter.
I think using the term cell-phone is what makes it unbelievable, because for the reasons above this is not how it is being used IMO. However, I don't disagree that there could be a communication device utilized if they are in fact advanced time travelers coming from a time of VLSI (or greater). The phrase: "If they have the ability to travel in time, then they can do 'X'" is a catch-all and could be used to any degree.
If the time traveler is from the future and he is discovered from this youtube video, wouldn't he have come to our time to stop the youtube video from being made!
135i MT JB/CR ///MSport