Joshua Tree is a Huge Cell Phone Dead Zone
Over 1.25 million people visit Joshua Tree National Park each year. However, I can tell you that you won't be able to make phone calls or receive data to your phone. No carrier has coverage throughout the National Park. You can drive almost 50 miles across the park and still not have one bar of reception. If you are lucky you might have a faint signal if you climb on top of a mountain or hill to receive a text. Good luck making a phone call if you are in an emergency situation.
I guess its one of the last remaining places where you can truly unplug.