...When investigators pulled the Chicago-bound packages off cargo planes in England and the United Arab Emirates Friday, they found the bombs wired to cell phones. The communication cards had been removed and the phones could not receive calls, officials said, making it likely the terrorists intended the alarm or timer functions to detonate the bombs.
"The cell phone probably would have been triggered by the alarm functions and it would have exploded mid-air," said a U.S. official briefed on the investigation, who like other officials spoke on condition of anonymity to discuss the case.
The official also said Tuesday that each bomb was attached to a syringe containing lead azide, a chemical initiator that would have detonated PETN explosives packed into each printer cartridge. Both PETN and a syringe were used in the failed bombing last Christmas of a Detroit-bound airliner.
http://www.nytimes.com/aponline/2010...ail-Bombs.html
I see from the URL these mail bombs are in the 'Politics' section of the paper.
These guys have years of experience building bombs with cell phone ringer circuits, looks like the timer function was used to initiate the detonation instead of a phone call. You might not know which plane the bomb is on but you can guess it will be airborne say, 18 hours after initial departure for a next day international delivery or 36 hours for a two day delivery. Especially if you've checked the timing with a dry run in September. Put in the tracking number for your Apple Store iPhone and you get the exact routing and times as FedEx or UPS bring your purchase around the world.
Bombing an airplane with a timer detonator is nothing new, remember the Sikh bombings within an hour of each other in 1985?