It's utterly insane to assume these 5k pagers were distributed before any of the users went on some international flight, so one of these must be true:

* they had very strict order to not carry pagers abroad - and 5k terrorists didn't break that rule once

* TSA and its equivalents are in on the attack

* the explosive nature of the modification did not show on the scans

If flying with pager was a thing, and I assume it was, how high was the likelihood of the attack striking in mid air? Maybe there's no signal in mid air. Was it a normal message that would get delivered as soon as signal was obtained, upon landing? Sounds like with 5k users the chances of some of them being on planes was far from zero.