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