Type the phone number into google, 904-239-3959, and you will get websites like 800notes, and whocallsme? Other people are getting the same threatening phone calls from that number. In most cases there isn't a legitimate debt and they use tactics that are against the law to try to get money from people. They claim they are from the fbi, and the police when they aren't. They threaten you with court and incarceration. They will say anything to scare you into sending them money. The companies they call from are not real companies. These people are like any typical scammers. That is why they violate the Fair Debt Collection Practices Act by making false claims, harassing people, and making threats. That is also why they won't send you any information about the so-called debt you owe.
Maybe if you show your boss this is a scam you can get your job back. This isn't your fault. You should go to the fbi website and file a complaint against John Parker, though it is pobably not his real name, and the phone number is from a prepaid cell.
I'm sorry this happened to you, and I hope you can get your job back.