This scenario is all but impossible. There has not yet been an actual incident (as opposed to an online legend) of a cell phone accidentally igniting anything anywhere. Apparently the warnings about cell phones as an ignition source are a carryover from decades-old warnings about the potential for sparks from old landline phones. There were a few instances of explosions many years ago that supposedly were triggered by cellphones; stories about cell phones causing those explosions went viral but the fact that the stories were debunked did not. Take a look at new gas pumps now – they no longer have a warning against using cell phones near them.
Of course, if you smell or hear gas outside that you think may come from a leak, for your own safety walk away before doing anything else, including calling 911.