Snopes does a much better job of explaining this than I would. Basic summary. Not only has there not been a single confirmable incident reported worldwide that can be attributed to cell phones, no one has been able to reproduce it in a lab. _If_ you take apart your phone and make an ignition switch out of it. and _if_ the station you go to has poor vapor controls, you _might_ succeed in igniting fumes from the pump.
Otherwise the only thing that's likely to even generate enough spark/heat to ignite fumes is if your phone's lithium battery ignites. There are a couple of documented cases of that, so if you're buying cheap batteries in china then in an exact perfect scenario at the pump, it _could_ happen.