California Code, Code of Civil Procedure - CCP § 700.170

(a) Unless another method of levy is provided by this article, to levy upon an account receivable or general intangible, the levying officer shall personally serve a copy of the writ of execution and a notice of levy on the account debtor.

(b) If a levy is made under subdivision (a) and payments on the account receivable or general intangible are made to a person other than the judgment debtor (whether pursuant to a security agreement, assignment for collection, or otherwise), the levying officer shall, if so instructed by the judgment creditor, personally serve a copy of the writ of execution and a notice of levy on such third person. Service of the copy of the writ and notice of levy on such third person is a levy on any amounts owed to the judgment debtor by such third person.