The bomb that demolishes the building is triggered by cellphone. Later McGarrett tries to use his phone but doesn't have a signal. Same for Kono who's trying to call them.
ETA: An explosion can damage or destroy cell repeaters, towers, and other infrastructure necessary for cell service. So it is very possible that an explosion detonated via cell signal can result in a period of little to no cell service in the area of the explosion.
ETA: An explosion can damage or destroy cell repeaters, towers, and other infrastructure necessary for cell service. So it is very possible that an explosion detonated via cell signal can result in a period of little to no cell service in the area of the explosion.