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.