We do have these signs (crossed cellphone) also at gas pumps where I live (Germany). As I stated above, I think these signs were introduced with the beginning popularity of cellphones and serve mostly as a precaution measurement because nobody knew 25 years ago if cellphones pose a harm to gas pumps when both are in use simultaneusly. In the same fashion that cellphones had to be shut off during the entire flight in an airplane. Gas pumps were there before cellphones. I personally witnessed people pumping gas and use cellphones at the same time and nobody cared. Depending on where you are from, your experience may differ.

Edit: typo, spelling