Couple Theorys Here:
A) We heard about the scud missile shipment or kimjongs train route (from our big ship in the pacific listening to all air coms)
B) Nextel uses satellites in space (thats how 2 way radio works).... no towers
Who knows what toys our boys have? No more than a reasoned feeling, but IMO the entire worldwidecomms network is virtually transparent to us, perhaps not all the time or with total comprehensiveness, but more likely for specific, targeted, chronologically sensitive applications.
Nextel PTT is digital trunked radio, which is, in turn, pretty close to a phone system - it has call setup, switches, etc. It most certainly does use terrestrial radio transcievers.