Mobile phones can cause sparks the reason that domestic petrol stations have the sign is primarily to protect themselves legally should anything go wrong.
The concentrations of fumes are much lower than at a refinery, so they have to take things very much more seriously there.
Incidentally, someone said "do not switch off but do not USE". This is because the act of switching the phone on or off will cause it to transmit, thus the possibility of a spark. Best to leave the phone inside the car and untouched. If it is on your belt and you get out of the car - best to place it back inside the car.
If some other accident occurred on the forecourt, the CCTV would already have shown you with your mobile phone.