We have mobile phones right now that don't need towers or satellites to communicate with each other. All a cell phone is, is a radio. Just playing the part of the Devil's advocate. As far as other users go, there could be other people there. If they have the tech to travel in time, then surely it would be possible for them to have cell phone looking communication devices.
(FBO) Cobb Stage 2+, AR Downpipes, Injen DCI, AA (Sport) FMIC, ER Chargepipe/Synapse BOV, Carbon fiber bits and Breyton GTS-R Wheels.