I think they intercepted a shipment of pagers, and inserted tiny amounts of explosive and some electronics to trigger when an appropriate signal was sent. Seems walkie-talkies too as some of them have just exploded today.

Hezbollah switched to pagers because they are receive only, so cannot be tracked, unlike mobile phones. They are also reported to have an extensive private land line setup. Presumably they communicate by sending code words over the pagers, which then tells the person to phone a particular number for instructions, or do some pre-arranged action.