Not sure about the US but in the UK mobile phones regularly transmit to the network to 'check-in' so if a call does come through it can be routed to the area of the network that the phone is and save time ( although I'm sure we are only talking milliseconds ). I hear this all the time when this causes interference over the radio in my car. This fact has also been used in the Law Courts to challenge alibi's by defendants who claim to be nowhere near the scene of the crime, with this log showing the relative position of their phone over the network.