Almost certainly a scam. Possibly from India or Pakistan. They might threaten you with arrest and so forth, which is a total scam.  Being in ordinary civil debt is not a crime and you cannot be arrested for it.

Just because a voice on the telephone claims you owe a debt does not mean you owe it
If a voice on the telephone claims you owe a debt:

1.    Demand the person give you his or her name, company name, and address
2.     Demand a debt validation letter by US mail.  E-mail no good.
3.    Do not pay  the collection agent or anyone else a  DIME  until the debt is validated.

Of course they won’t do this, no scammer ever will, so simply block their calls and ignore them.

If they are bothering you,

Report them:

http://www.consumer.ftc.gov/articles/0149-debt-collection  (Threatening arrest or criminal process in order to collect a civil debt is a clear violation of the Federal Fair Debt Collection Practices Act)

Also contact your local attorney general

If they continue to bother you, and you know you do not owe any money,  tell them that you know this is  a debt scam and that you are making a complaint to the police for extortion.  You might have to yell at the scammer and talk over him or her.  Once you get a copy of the police report and they call you again , just read out the  file number and the name of the PD or Sheriff’s office involved.   Tell them you’ll be glad to send a copy to them at whatever address the jack***es are working out of now.  If you can ever find them and they are located in the US.