Leave a message to call an 866 toll free number but don't who they are, who they are looking for - only that they are calling re collection of a debt that I know I don't owe anything that would require a debt agency to contact me.
You don't need a valid debt or a derogatory trade line on your credit reports to be hassled by debt collectors. They call third parties to alleged debtors all the time for permitted skip tracing purposes, or often pretending it's a trace to annoy and shame an intended target whose location is already known.
FTC and CFPB material on US federal collection law:
http://www.ftc.gov/os/statutes/fdcpajump.shtm
http://www.consumerfinance.gov/askcfpb/search ... debt-collection