It is fraud - these scammers can't even get the lingo right!  If your sister was being sued, she would receive a court summons, not a subpoena, via process server (or in some cases, certified or registered mail).  She also wouldn't get a heads up that she'd be served either.  Your caller has also violated your sister's privacy by disclosing information to a party other than the intended recipient.  Next time he calls, tell him that he's been reported to the FTC, FBI and your state's Attorney General.  

http://800notes.com/forum/ta-86217073a9c8dad/ ... 077595690349410
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http://www.consumer.ftc.gov/articles/0258-fake-debt-collectors
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http://www.deadiversion.usdoj.gov/pubs/pressreleases/extortion_scam.htm
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Also read up on the laws and your rights:

Harassing calls from a debt collector? Here is what you need to know
http://800notes.com/arts/Jb8EW-eDhQA/harassin ... ou-need-to-know

http://www.ftc.gov/os/statutes/fdcpajump.shtm

CFPB Q&A on debt collection practices
http://www.consumerfinance.gov/askcfpb/search ... debt-collection

http://www.consumer.ftc.gov/articles/pdf-0096-fair-credit-reporting-act.pdf
http://www.consumer.ftc.gov/articles/0149-debt-collection
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Follow the steps in what you need to know. A legitimate debt collector agency should not have a problem with giving you a real business name and address you can google and verify via google street view, BBB etc.
For extortion Scam Operations, file reports with the FBI, your state attorney general and Department Of Justice,FTC & FCC..

http://www.nw3c.org/
http://www.ic3.gov/default.aspx
http://www.fbi.gov/
https://www.ftccomplaintassistant.gov/#&panel1-1
https://esupport.fcc.gov/ccmsforms/form1088.action
http://www.consumerfinance.gov/complaint/

And folks should consider looking at maybe getting call blocking device or phone..
http://www.amazon.com/s/ref=nb_sb_noss?url=se ... s=call+blockers