Cell phones can not receive signals in many areas of the Sierra Nevada and other mountainous regions throughout the state.
The mountains often block signals. While outside mountain regions, cell phones can be a great help in an emergency, you should never depend on them for help in a hiking emergency in the mountains. Even if you are able to reach authorities, you still must identify where you are - which can be tough if you're lost! And even if you can identify where you are - on which mountain and trail - it can be hours before rescuers reach you. If you do reach authorities, be prepared to tell them your location, the nature of the emergency, your cell phone number and call forwarding information if you are out of your regular coverage area. Be specific.

Remember: technology is NEVER a substitute for knowledge and experience. You must know how to rescue yourself.