If it was from 2007 the statute of limitation in CA has passed. They cannot sue you but you cannot dispute that you do not owe it if you do. You can asset the statute of limitations defense and ask that they not contact you anymore.
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The statute of limitations has run, They cannot file a lawsuit to enforce/compel payment of this debt.
Tell them so.
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Do not assume that junk debt buyers claiming to own old debts actually own them. In many cases, they do not. Debt buyers can rarely prove anything regarding claimed debts, including their ownership of them, your liability, or the amount.
If the debt is out of statute, you should direct them not to contact you further. Have an attorney review the letter you got; attempting to collect out of statute debts without disclosure of that fact may be an Fair Debt Collection Practices Act violation.
If the debt is within statute, send a letter asking for proof that the debt buyer owns the debt. I doubt if you will get anything back. Then, if they do sue you, you have a defense -- under the Uniform Commercial Code you are entitled to reasonable proof (such as a copy of an assignment referring to the particular account) that the assignee debt buyer owns the debt.