What the other person owes is not any of your business. While it is not illegal for you to make the call as long as you don't give false information to the issuer (like lying about your identity) federal and state privacy laws would not allow the issuer to give you that information without the cardholder's express consent. If you do lie to get the information, you may be committing one or more identity theft or fraud related crimes. My suggestion is not to make that call. The saying "curiosity killed the cat" comes to mind here.