Classic debt scammer ploy.   No sheriff will serve you with any papers unless there is a case filed against you, or you are a witness in a criminal trial.  If there was a case filed against you for breach of contract, the attorney for the other side would have already contacted you by mail to work out some sort of negotiated settlement.

You'd already know about it and you wouldn't find out about it from some debt scammer.  This is all total nonsense. These are classic ploys of New York state kitchen-table debt scammers.  Shoving meaningless legal terms at you by way of a cold call and hope that  you'll be scared into giving them money.  Don't fall for it.  But don't talk to them and don't return their calls.

Fraudulent or Zombie Debt Collectors—Things to Know

If you have received a call stating:

1. You have defrauded a financial institution
2. You owe an unpaid payday loan
3. You have a warrant due to an unpaid loan
5. You will receive a court summons
6.  Authorities are coming to arrest you
7.  Anything similar to above,
a.   Do not give them any information or verify any information for them
b.   Tell them you know they are a scam operating illegally and that you are   filing a complaint with the  appropriate authorities
* File a complaint with the FBI on their Internet Crime webpage     http://www.ic3.gov/default.aspx
* File a complaint with the Attorney General for the state you live in
The more information and complaints there are, the better the chance of getting these criminals to stop (or at least reduce the frequency).    

They continue to bother  you, tell them you are going to the police to make a complaint of extortion.