Never use your mobile phone at the petrol pump, because the phone can spark and ignite the fuel vapours, blowing up the whole station. The warning stickers are right there on the bowsers, so it must be a real and present danger.
There is no verified case anywhere in the world of a mobile phone igniting petrol vapour at a service station. The fear took off in the early 2000s through viral emails and got baked into signage out of caution, but investigations by fire authorities and fuel companies have never found a single confirmed phone-caused fire; the energy from a normal phone is not enough to ignite fuel vapour under realistic conditions. The hazard that is real, though still rare, is static electricity. People who slide back into the car seat while fuelling can build up a static charge, then create a spark when they touch the metal nozzle or car, and that has caused a small number of fires, almost always involving someone getting back in and out of the vehicle mid-fill. So the genuine advice is to stay outside the car while fuelling and touch metal to discharge static, not to fear your phone. The phone ban is essentially folklore. The seat shuffle is the actual culprit, and a far less catchy warning.