There have been many cases of gas station explosions in recent years, and the cause of the large majority of them have been a buildup of static electricity on the person's body. This is most often caused by re-entering the vehicle once pumping has started. When exiting the vehicle, they then return to the nozzle, only to discharge that static charge either on the nozzle itself or against the body of the car. This static spark ignites the gas vapors, causing the explosion and fire.
There has never been a documented case of a cellular phone igniting gasoline vapors. Think about it: when was the last time you saw a cell phone creating an electric arc? Have you ever touched the antenna and caused a spark? Has your antenna ever sparked while you were talking?
You have never seen that happen because those two energies are slightly different. The antenna produces radio frequency radiation, which does not produce sparks. Sparks are most often caused by static electricity, or high voltage arcing between contacts. The electricity from the battery is not at a high enough voltage to cause a spark or create an arc between the battery and the phone if they lost contact with each other. If that were true, you would have to disable every item near a gas pump that used a battery, including watches, pagers, pacemakers, hearing aids, etc.
So why then the big push by the major gas stations to place warning labels on gas pumps? Let me answer that with a question. Have you ever seen the warning label on a chainsaw that says "Don't attempt to stop the chain with your hand"? Or the label on a child's Superman costume that reads: "Warning: wearing this suit does not enable you to fly." Now why would any company in their right mind need to place warning labels like those? Perhaps because someone has tried it, found that it did not work (the hard way), and told the manufacturer. To protect themselves from frivolous lawsuits, they place the warning label. After all, a label is usually cheaper than a lawsuit.
However, does the incident actually have to occur before a company will place a warning label? The public concern over the possibility of injury, no matter how slight (or in this case, nonexistent), could be enough to cause the company to act, lest the company be portrayed as disconnected and uncaring. And there is enough public concern in the area of cell phones that the companies are placing labels.
But we know that the chance of a cell phone sparking a gas fire are so remote that it is almost not possible. Then why go through the expense to place labels? It's a response to public concern. We cause the gas station companies to place those labels. It's our concern about the unknown that causes them to act.
So what to do? While I don't think it's necessary to stop your conversation while you pump, it doesn't hurt to make others feel better by hanging up until you're back in your car. But please, pump your gas; don't stop your pacemaker.