On Isla Nublar, there was a radio network, that extended through the island. Not only were the Land Cruisers equipped with radios, but also the gas powered Jeeps as well. The radios in the tour cars allowed guests to talk to each other through the radio sets. Both the Jeeps and the Tour cars also had the capability for their radio sets to talk to the control room, as well as the people in the control room to listen to what the guests might be saying in the cars.
Due to the interference of the storm, radio communication became increasingly difficult, as messages on the radio ended up static. During the Tyrannosaur breakout, the radio in the Land Cruiser was destroyed, thus Alan Grant left it there instead of taking it with him. In both the tour cars and the jeep, the radios were able to be removed by means of a detachable handset.
Jurassic Park by Michael Crichton page 134, 144, 147-149,160,164, 166-167, 172, 174, 181-182, 185-187, 192-193, 218-219(First Edition, Thirty Fourth printing)