I think the warning to turn off your mobile at the gas station originates from when mobiles first came on the scene and had components that could potentially cause an ignition. Today's mobiles no longer have these components.
And I don't think it's possible for the mobile signal in the air to ignite the fuel vapor.

In the UK the petrol pumps are designed in such away that you have to keep the pump lever pressed in, so you can't walk away and leave it refueling as you can here in Qatar. I believe this greatly reduces the chance of static electricity causing an ignition.

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