I am not sure what you mean by lpg, but, if it means Liquefied petroleum gas (as I inserted in your question), then the show Mythbusters had a show about this one day. I copied the link to a youtube video here:
In short, can it cause a fire? Sure, if used for such a purpose. But, according to the video, under normal use, according to Mythbusters, a cell phone just does not ignite gas.