If you use your cell phone while pumping gas and do not build up static electricity then nothing will likely happen.
Although it's not advised to use a cellphone while you're pumping gasoline because if you do happen to build up static electricity then the static electricity could cause the gasoline vapors to explode.
The cellphone can create static electricity along with the radio waves the cellphone uses and then gasoline vapor can explode.
I've seen some people talking on cellphones while pumping gasoline and nothing bad has happened but you still should not do it.
Also do not get in and out of your vehicle while pumping gasoline because it can build up static electricity as well and then the car could likely explode.
It's not worth the chance.
Stay out of your vehicle while the gas is pumping and keep an eye on it just in case the pump doesn't shut off.