It may or may not be legitimate. Unless they are willing to provide you with verification of who they are and who they represent, I suggest you simply not interact with them.
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It is unlikely an attorney, even a debt collection attorney, would phone from a blocked number. If you speak to him again, ask him what his Bar number is. If you can get a person's name or Bar number, you can run him/her on the California Bar website: www.calbar.ca.gov.
There is a nationwide company called "Imperial" which does credit ratings and debt collection. If they claim you owe a debt, they must send you proof of that debt. They can't send you a fantasy certified mail letter that demands payment for a certain amount of money. You are entitled to all the information to determine whether it's already been paid, whether it's even yours, or what the debt is for.
Tell them to serve you the papers, but you will require them to adhere to all the rules of the California Civil Code sections 1785, etc including the California Fair Debt Collection Practices Act. They'll hiss at you like snakes but they probably won't serve "papers."
Tell them also that if you don't receive proof of this debt in the form of the creditor's invoice to you (or however you were notified that you owed money) you will file a complaint with the Federal Trade Commission as well.