From the article:
Revelations that every phone and mobile device can be remotely used as a bug (at one point, Snowden hides beneath a cowl to protect his keystrokes) crank up the tension further still; when a mobile phone went off in the early screening I attended, the audience’s annoyance was tempered by all-too-nervous laughter. I’m sure I wasn’t the only one who went home and took the battery out of their mobile.
Taking the battery out does no good. There is another, internal, unremovable, battery that tracks the location even when the battery is taken out.
The only way I know of to truly turn "off" a cell phone is to put it in a mylar bag, like a potato chip bag. The metal in the bag bag will intercept transmission from the phone. Unfortunately, the bag will also prevent transmissions to the phone.