California Code, Code of Civil Procedure - CCP § 488.470

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

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