Los Angeles Times:
Debt collectors calling? You have rights.
-- The debt collector must tell you -- within five days of initial contact -- the amount you owe, the name of the creditor and how you should proceed if you think you don't owe the money. If you dispute the debt, send the collection agency a letter within 30 days saying so. Once the debt collection company receives your letter, it must stop trying to collect until it sends you written proof of the amount you owe.