Does the iPhone 4S generate a spark or detectable electric signal of any kind when receiving a call?

In my spare time, I am a volunteer for the fire brigade. Yesterday I was attending a lessons about preventing fires, and we saw a movie about a lady fueling her car while receiving a call. And "boom"! The fuel, and later the car, exploded because her cell phone got a call. The teacher told us it was because her cell phone gave an electrical signal (a kind of *spark*) and by so light up the whole place. Since I tend to carry my iPhone 4S in my pockets all the time, I'm concerned about the risk of entering a building filled with some gas. If I get a call when I'm in such a building, I'm afraid I won't be around to answer the call anymore. :) So, the teacher told us there are special cell phones which are *explosion proof*. These phones don't generate a spark or detectable electric signal of any kind when receiving a call. Is the iPhone 4S such a cell phone? In other words, is it safe to carry an iPhone 4S when entering a building filled with gas?