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?
To cut it short: This assumption is false.

Cellphone are not able to cause high voltage sparks required to light gas. Probably only if short-circuited.

Mythbusters investigated this problem. Their answer was that such accidents were caused by static charge on the person, not by cellphones.

This is important to notice as
"Petrol has a low electrical conductivity. This can cause a charge of
static electricity to build up as it flows through a pipe."[1]
When both the petrol and the person are charged, there is a potential spark discharge in the gap between.

Edit:


A BBC news article mentioned that research has been conducted on the matter by Dr Adam Burgess of the University of Kent. The publication is called:
‘Phantom Risk: The Curious Case of Mobile Phones, Fire and Body Static,’ Health, Risk and Society 9 (1) 2007 ISSN: 1369-8575 and available here.
[1] The Australian Transport Safety Bureau has published a paper called 'Static Fires at Petrol Stations' which is publicly available for download.