CA Ins Code §10123.135(h)(4)

(4) Communications regarding decisions to approve requests by providers prior to, retrospectively, or concurrent with the provision of health care services to insureds shall specify the specific health care service approved. Responses regarding decisions to deny, delay, or modify health care services requested by providers prior to, retrospectively, or concurrent with the provision of health care services to insureds shall be communicated to insureds in writing, and to providers initially by telephone or facsimile, except with regard to decisions rendered retrospectively, and then in writing, and shall include a clear and concise explanation of the reasons for the insurer’s decision, a description of the criteria or guidelines used, and the clinical reasons for the decisions regarding medical necessity. Any written communication to a physician or other health care provider of a denial, delay, or modification or a request shall include the name and telephone number of the health care professional responsible for the denial, delay, or modification. The telephone number provided shall be a direct number or an extension, to allow the physician or health care provider easily to contact the professional responsible for the denial, delay, or modification.