California Code, Vehicle Code - VEH § 24018

(a) Every transit bus operated by a motor carrier, whether that motor carrier is a private company or a public agency, that provides public transportation services shall be equipped with a two-way communication device that enables the driver to contact the motor carrier in the event of an emergency. The two-way communication devices shall be maintained in good working order.

(b) For the purposes of this section, “two-way communication device” is a radio, cellular telephone, or other similar device permitting communication between the transit bus driver and personnel responsible for the safety of operations of the motor carrier, including, but not limited to, the motor carrier's dispatcher.

(c) This section does not apply to buses operated by a school district or on behalf of a school district.

(d) The commissioner shall upon request grant a nonrenewable one year extension to any motor carrier to comply with the requirements of this section.

(e) Nothing in this section shall require a motor carrier to replace an existing two-way communication device that currently meets the requirements of this section.