Where's Waldino?
Can you see him? Here's a hint: look behind the hoses. See him now? Aaah yes, that would be him talkin' on his handy-dandy mobile chatter box.
Although the chances of starting a fire this way are distinctly remote, the fact is that your cell phone just wasn't designed to be used in an ignitable environment. Your best bet is to not use the thing.
One other point to consider is that when you are talking on the phone, you are not paying full attention to the task at hand, making it more likely that you mess up. Think about it...
P.S. Don't talk on your cell phone while driving either! Bumper sticker quote of the day: "Hey idiot, you're driving a car -- not a phone booth!"
Department of Defense (US Navy Safety Program)