IIRC it all started when a delivery tanker didn't earth the truck before connecting the hose to the underground tank - there was flash but no real fire - I think they may have modified the design so the connection automatically earths these days
Certainly nothing to do with a mobile phone but it gave people something else to put up a DON'T DO THIS sign........
A cynic might even think that it was done to speed up people pumping gas rather than talking to their nearest & dearest........................