So, if the method was to substitute the batteries for ones containing explosives, then how were they triggered simultaneously? Wouldn't this also require some

So, if the method was to substitute the batteries for ones containing explosives, then how were they triggered simultaneously? Wouldn't this also require some additional HW/SW in the pagers to trigger the devices? Otherwise, if it was just battery terminals connecting to the battery, how would a remote signal trigger them? Or maybe it's as simple as the adulterated batteries containing timers and thus not needing external triggering?