Check the statute of limitations for breach of contract in your state. If the debt is stale due to the statute of limitations running, the debt collector is violating the Fair Debt Collection Act. Also, study up on the FDCA. Debtors have a lot of rights to be free of harassing debt collection practices.
Medical providers generally do not report to credit bureaus. But if they sell the debt to a collection agency, the debt will more than likely be reported. Once it is reported, it is very difficult to dispute it with the bureau and get it taken off. The bureaus are pretty reluctant to take action based on a consumer complaint. They bureaus make most of their money from businesses that report to them. They do not like to bite the hand that feeds them. But there are remedies under the Fair Credit Reporting Act for consumers who believe that the negative reporting is not accurate. Study up on that one too.
Finally, you need to be careful that you do not get sued. If you get sued and lose, the debt collector, depending on your jurisdiction, may be able to get attorney's fees awarded and may be able to garnish your wages.