That doesn't follow. You could have a timer that causes the pager to vibrate as if it had received a message or an alarm had rung. That would make the attack simpler, in that one wouldn't also have to compromise (or risk leaving traces in) the phone system to activate thousands of pagers.
I'd imagine a backup timer, with the ability to trigger early if required for strategic or tactical purposes.
I almost surprised this wasn't coordinated with (or saved for) an incursion into Libanon. That seems to be something Israel wants to do, and this would be a great way to disrupt the defense at the most critical moment.
Seems easier to install a transmitter and GPS in the supposedly receive-only device. Then it could actually track people and show where they had been. It could store up readings and only transmit while the device was in motion at around walking speed, with signal strength above Y. That means the person was probably outdoors and moving, thus probably ot being swept by bug detectors. Well maybe not any more, now that everyone is thinking that way ;).
Pagers just simply have an address (called a Cap Code) to receive messages. It's like a mailbox number. A pager can be programmed with usually up to 4 Cap Codes at a time.
If I was speculating on what happened, I would bet that the pager had 3 Cap Code addresses programmed, the mailbox cap code the owner of the pager expected to have for receiving messages, a cap code that was the same programmed in all the pagers to that functioned normally to received messages, and then the 3rd cap code programmed in all the pagers that when receiving a specific message triggered the explosive.
The folks responsible simply sent a message to the 2nd cap code to get all the pagers to go off, presumably to get the targets to get the pagers out and look at them, and then immediately the trigger message next to the 3rd cap code to detonate the explosive.