You and me both. Although the bridge is pretty massive as I recall, you'd need a lot of explosives to bring it down.
Still a good sized truck bomb should do it if you set it off in the right place so that it severed one of the main cables - but this could be done without cell phones, unless they were planning to leave the truck on the bridge as "broken down" and walk off, then set off the explosion using a cell phone once they were safely on land. Seems like a somewhat shakey idea - is the cell reception reliable enough in the middle of the bridge for that to work? Setting off the bomb on one of the approaches would be easier but would do less damage.
However this would only take a couple of cell phones, a half dozen at most. My bet is that the plan was for something much bigger and more spread out - bombs in many places perhaps, or a major drug operation. What else would you need hundreds of phones for?!
I agree with you on that one.