After July 1, 2008, California drivers may be issued citations if caught talking on a cellular phone while driving. The Department of Motor Vehicles has a nice page dedicated to the new laws as well as a short video clip. There is no grace period for this violation.
Drivers 18 and older must use a "hands-free" device while driving. There are exceptions for emergency use and for certain persons such as emergency workers in the performance of their duties. The violation is reportable by DMV but not a point on your driving record. Check with your insurance company as to how a violation may or may not affect your policy. With penalties, the fines start at about $76. A second offense hits $190. This is a primary violation and you may be stopped if the officer believes you are using the cell phone while driving. "Dialing" is not prohibited, but strongly discouraged. (CVC 23123)
Young drivers, under 18 years of age, are prohibited from using any communication device while driving. This not only includes cell phones, but also pagers and texting devices as well. This violation is secondary. DMV reports that statistics show young drivers are more likely to be involved in a collision due to inexperience. This is why this section is more restrictive. (CVC 23124)
These laws are on the books to improve your safety. Please comply with these sections and "Get Off the Phone" while driving.