If the statute of limitations has expired, the collector cannot sue you but can continue to contact you about the payment.

What is the statute of limitations?
In the United States, the statute of limitations on debt collection by state or statute of limitations is a rule that establishes a time limit within which a creditor can sue you for the payment of a debt. The length of time a creditor has to sue you for an unpaid debt varies from state to state. In some states, it is four years. In other states, it could be more. The time limit may also depend on whether your agreement with the creditor is in writing or not, and whether the debt is of a special type, such as a revolving or open account.
If the time limit for suing on the old debt has expired under your state's statute of limitations, that doesn't mean that a creditor or bill collector should stop contacting you about it. They can ask you to pay the debt, but they cannot sue or threaten to sue you for it.

Don't fall into the trap of reviving your prescribed debt.
If you are contacted by a debt collector about overdue debt, be very careful what you tell the collector. If you say or sign anything that can be considered as a recognition of the validity of the debt (that is to say, agree that should the debt even if the statute of limitations to sue expired ha), it is possible that the has "revived", waived or extended. Even if you make an agreement with the collector of that bill to pay the old debt, you can also revive, waive, or extend the statute of limitations.

Potential violation of the Fair Debt Collection Practices Act (FDCPA).
If you are not sure whether your debt is due under your state's statute of limitations, and you ask the collector if that debt has a set term, the FDCPA ( Fair Debt Collection Practices Act ) says the collector must tell you the truth.
If your debt is overdue, but the debt collector has threatened to sue you or take other legal action to pressure you to resolve that debt, you may have violated the FDCPA. The FDCPA prohibits debt collectors from threatening legal action on prescribed debts.

California Laws Regarding the FDCPA.
From January 1, 2019, l as collection agencies in California must inform the debtor if the debt has prescribed. The collector must include the notice in the first written notice sent to the consumer after the statute of limitations is approved.

Check your credit report or history.
If communicating with you for a debt prescribed, you should check your credit history. Often times, bill collectors or creditors report negative information about the debt as recent information. This could be a violation of the Fair Credit Reporting Act.