There are a couple myths about cell phone usage during storms producing lightning. Electronic devices such as cell phones do not attract lightning on their own. But they can contribute to the severity of injuries sustained if a user is struck by lightning while holding a cell phone. Knowing how lightning works also helps to understand how lightning strikes can happen.
A lightning strike that hits the ground is called a "cloud-to-ground" or "CG" flash. This kind of flash happens in stages that take place over the course of a few milliseconds. Lightning gets its crooked shape by the electricity changing its path as it travels from the cloud to the nearest metal object or tallest object on the ground. Right before making contact, the part of the lightning that is closest to the ground surges in strength, to around 10 million volts. This attracts electricity from the grounded object, and the two charges actually connect with each other some 30 to 40 meters above the ground. This connection between the electricity traveling down from the sky and the electricity summoned from the ground is what produces the flash you see as lightning and hear as thunder.
It's true that a cell phone's signal is not strong enough to draw lightning down from the sky, but because the body of a cell phone is made at least partially from metal, it can still conduct the massive amount of electricity carried in a lightning bolt. You also don't have to be directly struck by lightning to be affected by it. Lightning can hit a nearby object and jump over to you, or to your cell phone, and thereby cause what's called "flashover," which is the external travel of electricity over your body. This can result in burns. If there is any kind of metal inside your body, the electricity can travel into your body and cause internal damage.
Lightning travels downward once it makes contact. Using a cell phone indoors is safe only if it is not plugged into a charger. If lightning strikes a house, and you're using a cell phone that's plugged into an electrical outlet, your chances of electrocution are much higher.
Always take precautions when using electronic devices while in bad weather, including cell phones, wireless talk devices and even MP3 players with ear pieces. Avoid using your cell phone until you are indoors, and don't use it at all if you are in an open area, such as a field or park, during a storm. Lightning is attracted to the tallest object in an area, which could end up being you and your cell phone, if used in the open during bad weather.