If you are not the person the debt collector is looking for – for example, if your name is the same as the person who owes the bill – explain the mistake in writing as soon as possible. You may be asked to provide a drivers’ license or other proof of identification to show that you are wrong person.
Perhaps you should send them a copy of your phone bill, showing the number and your name. When you do this, consider asking them to stop calling you, or to stop communicating with you altogether:
You may also ask a debt collector to contact you only by mail. Make your request in writing, send it by certified mail, and keep a copy for your records. You also have the right to ask a debt collector to stop contacting you entirely. The request must be in writing, and you should send it by certified mail and keep a copy for your records. If you make this request, the debt collector can only contact you to confirm it will stop contacting you and to notify you that it may take action against you. (15 United States Code section 1692c.) Remember, though, that if you request no contact at all, the debt collector may take you to court and may still report your debt to credit reporting agencies.
Then, if they continue to call (the hyperlinks are in the source):
If you believe that a debt collector has violated the law, you can file a complaint with the Attorney General's Public Inquiry Unit. Consumer complaints are valuable to the Attorney General’s Office because they alert us to debt collection issues and other issues that California consumers face. We may forward a copy of your complaint to the debt collector; however, please be aware that the Attorney General’s Office cannot represent individuals or give legal advice.
You may also wish to file a complaint with the Federal Trade Commission (FTC). This agency enforces the federal Fair Debt Collection Practices Act. The FTC may be contacted by mail at Consumer Response Center, Washington, DC 20580-0001, or by telephone at 1(877)-FTC-HELP, or at FTC Complaint Assistant.