If they show up do I have to take them an what could happen

Sounds like a typical scammer.  It's illegal to threaten people with fraud, charges and service of "papers" - per the FDCPA (Fair Debt Collection Practices Act).  NO ONE gets a heads up they'll be served - the process server just shows up, unannounced.  If you had committed check fraud, guess what?  The ORIGINAL vendor that you wrote the check to would have come after you.  And the decision to charge someone with check fraud lies with the original vendor and the local DA - not with subsequent buyers of the debt.  

Tamianth, one of the regulars here, posted some excellent information on another thread about the same kind of call.  Info in the links:

Per Tamianth:

One important thing to remember if these losers have any of your personal information such as your social security number, date of birth or drivers license make sure you register with identity protection notify the credit agencies as well as your banks!!
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credit reports:  Experian, Equifax, TransUnion
http://www.consumer.ftc.gov/articles/0155-free-credit-reports
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How to see if your on the chex system list:
http://www.wikihow.com/See-if-You-Are-on-Chexsystems-List

http://www.ckfraud.org/penalties.html

http://www.ckfraud.org/ucc_highlights.html

http://www.ckfraud.org/depository.html

http://www.consumer.ftc.gov/scam-alerts
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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

http://800notes.com/arts/Jb8EW-eDhQA/harassin ... ou-need-to-know
http://www.ftc.gov/os/statutes/fdcpajump.shtm
http://www.consumerfinance.gov/askcfpb/search ...
http://www.consumer.ftc.gov/articles/pdf-0096-fair-credit-reporting-act.pdf

I keep getting a call from this number telling me I owe money on a old check from years ago said if I don't pay its check fraud an  someone gone Come to my house an serve me with papers is this a scam