Being on the phone is a myth anyway. It doesn't cause fires in petrol stations.
It was a much higher risk with the old analogue phones than the modern digital ones.
In a fuel or fuel vapour environment, all equipment is supposed to conform to many regulations. Mobile phones do not comply with the regulations hence the warning. Intrinsicly the pumps are designed to minimise risk because of the public access and hot car engines/exhaust etc so most of the regulations are exempted because of practicality but obviously some safety regulations must be in place.
A mobile phone transmitter is capable of creating an High-voltage, low energy spark not dissimilar to that of the piezo spark in a cigarette lighter - this is not just a hypothetical risk, although it may be considered unlikely.