"They wouldn't have to "conscript" or confiscate the handsets, just turn off the towers. Cell phones always communicate via the a near by tower. Two cell phones a block away won't communicate if there is no tower. (How could they bill you?)"
I don't know anything about the technology here, but can a cell phone alone be used as a transmitting device to communicate directly with a detonator?
My guess is they use the cell phone as part of the detonator, and merely call it up to detonate, just a guess though.
Anything is possible, depending on how much work you want to put into it. Analog phones are just FM radios, so, depening on the phone you use, a scanner could recieve a signal and close a relay to set off a bomb. GSM would be harder, but if you could find an otherwise unused GSM frequency band in the area, you could do a very crude radio detonator with a GSM handset. Personally, given the blast radius, I'd prefer the high power output of an analog handset.
The bottom line is, though, for a sploit as big as this, you could have a radio custom built. That was a pantload of explosives, and even if they came from a Chinese supplier to Wal Mart, had to cost more than a big bag of tube socks.