Det. Messer is on a train talking on a cellphone to Mac. He hangs up by saying something like "I'll talk to you later, I think I am going to loose the signal". The problem is he is in the subway underground, you cannot get a cellphone signal once underground in the NY subway system you can only get service if the train is on the surface (above ground) this is true even if the train stops partially underground. A few major cellphone companies are trying to bring service to the subways but it has not happened yet.