Cal. Pub. Util. Code § 766.5
The commission shall investigate the practices of every telephone corporation in billing its subscribers for telephone calls, including, but not limited to, whether a corporation is unable to determine whether any telephone call was not completed and as a consequence may charge the subscriber for that uncompleted call. If the commission finds that any corporation does not have the capability of determining in every instance whether a call placed by a subscriber was completed, the commission shall require the corporation to furnish written notice, in a form and manner approved by the commission, to its subscribers of its billing practices, including, but not limited to, its practice of charging for calls placed but not completed.
Ca. Pub. Util. Code § 766.5