Impersonating a Officer if the Law is a criminal offense all on its own. I think the real US Marshall's have better things to do then run around calling people about a civil matter. I'd also report these calls to them as well.
I got the same call man claimed to be a marshal and gave me a number to call to settle with some guy I tried to call the so called (marshal) and it goes directly to voicemail every time pretty sure a marshal would be available eventually