If the collection agency contacts you first by phone, they must follow up with a written notice within 5 days.
When the agency sends you the written notice, you should respond immediately. The collection agency should get your letter within 30 days of when you got their first notice. Keep records of all your communications with the collection agency.
You should respond to the collection agency in writing to dispute the debt if you do not think you owe the money. You should ask for verification of the debt. If you have proof you do not owe the debt, attach it.
You should send this letter by certified mail, return receipt requested. Keep a copy of the letter, evidence, and the certified mail receipt for your records.
*You can use sample letter # 3.
To learn more about how to respond to creditors, read Debtors' Rights: Dealing with Collection Agencies.