This is a common ploy by criminals masquerading as debt collectors who are attempting to extort money from people by scaring you into believing that you will be criminally charged, go to jail, lose your driver’s license, have wages garnished, be sued, and a variety of other variations on this,  all for an alleged or nonexistent debt.  One of the tricks they use is to call your work place, friends, or relatives repeatedly in an attempt to shame you into paying.  They also use the “process server” ruse who calls and claims he is going to serve you, but then says you could avoid it by calling another number.  Please not that process servers do not ever call ahead.  That way you can’t dodge them.

Read up on your rights here, get template letters to send and also make a complaint at this government site: http://www.consumerfinance.gov/

Also file a complaint with your State Attorney General's office.
List of State AG’s offices:  http://800notes.com/faq/attorney-general