That sounds like a criminal debt collector trying to scare you into paying their extortion fee.  It would be helpful if you could post the name of the company and any other information if you have it.
This is a common ploy by criminals 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.  They also use the “process server” who calls and claims he is going to serve you, but then says you could avoid it by calling another number.  Now why would a “process server” want to do you, a stranger, a favor, and at the same time not get paid to deliver the papers?

Federal law (FDCPA) requires them to send you a letter (US MAIL ONLY) within 5 days of their first contact that contains their name, physical address, the creditor’s name, and the amount of the alleged debt.  It also must contains “mini-Miranda” telling you that it is an attempt to collect a debt and that all information will be used for those purposes.  The one other important thing that this letter must also have in it is that you have a right to dispute the debt within 30 days receipt of the letter and if you do so, all collection activity must be stopped until the debt is verified.
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://consumerfraudreporting.org/stateattorneygenerallist.php