California Code, Labor Code - LAB § 3761

(a) An insurer securing an employer's liability under this division shall notify the employer, within 15 days, of each claim for indemnity filed against the employer directly with the insurer if the employer has not timely provided to the insurer a report of occupational injury or occupational illness pursuant to Section 6409.1.  The insurer shall furnish an employer who has not filed this report with an opportunity to provide to the insurer, prior to the expiration of the applicable time period specified in subdivision (b) of Section 5402for rejecting a claim, all relevant information available to the employer concerning the claim.

(c) In establishing a reserve pursuant to a claim that affects premiums against an employer, an insurer shall provide the employer, upon request, a written report of the reserve amount established.  The written report shall include, at a minimum, the following:

(d) If an employer properly provides notification to its insurer pursuant to subdivision (b), and the appeals board thereafter determines that no compensation is payable under this division, the insurer shall reimburse the employer for any premium paid solely due to the inclusion of the successfully challenged payments in the calculation of the employer's experience modification.  The employee shall not be required to refund the challenged payment.