Cal. Unemp. Ins. Code § 931.5

(a) Except for Part 2 (commencing with Section 2601) of this division and Division 6 (commencing with Section 13000), any third party which makes a payment included in the term “wages” solely by reason of subdivision (a) of Section 931 shall be the employer with respect to those wages unless the third-party payer notifies the last employer, who is a member of the plan and for whom the services were performed, within 15 days of payment, and provides all of the following information to that last employer:
1. (1) The name and social security account number of the recipients of the wages paid pursuant to subdivision (a) of Section 931.
2. (2) The amount of gross wages paid pursuant to subdivision (a) of Section 931.
(b) The special rule prescribed by this subdivision applies to the payment of sick pay made by a third-party payer, such as an insurer, under a contract of insurance pursuant to a multiple employer plan that is obligated to make payments for sick pay to employees of participating employers. If the third-party payer provides the plan with the notification required by subdivision (a) within the time required, the plan, not the third-party payer, shall be treated as the employer under subdivision (a). If within six business days after receipt of the notification the plan similarly notifies the last employer for whom the services are performed, and who is a plan member, that employer, not the plan, shall be required to report and pay the contributions due with respect to the wages.
(c) The employer, as determined by subdivision (a) or (b), shall pay contributions, required by this part, except as provided in Sections 984 and 986, and shall comply with the requirements of subdivision (a) of Section 1088.
(d) When an employer receives the notification prescribed in subdivision (a) or (b), the wages described therein shall be deemed paid when the notice is received.
(e) The director shall not make an assessment pursuant to Section 1126, 1127, or 1137 to assess employee contributions required by Sections 984 and 13020 on third-party sick pay as described in subdivision (a) for the period from January 1, 1987, through the date on which this subdivision became effective.
(f) Except as provided by Section 1176 and Section 19301 of the Revenue and Taxation Code, no refunds may be made for employee contributions required by Sections 984 and 13020 paid on third-party sick pay as described in subdivision (a) for the period from January 1, 1987, through the date on which this subdivision became effective.