It's a scam, of course; they're just thieves trying to steal your money. There is no US National Bank, and even if there were such a bank, banks don't have the power to charge people with crimes in the United States (that's the job of prosecutors, grand juries and attorneys general.) These morons think private businesses in the U.S. can issue arrest warrants and charge people with crimes because they're foreigners calling from overseas and have no idea what the law is here. Their ludicrous attempts at using meaningless phrases like "downloading warrants" would be funny if it weren't for the fact that they are trying to steal your money.
Just remember this: no debt collector (let alone criminals pretending to be debt collectors) has the authority to have anyone arrested for anything. Also, it is ALWAYS a scam whenever someone calls you and demands that you pay them money to prevent your arrest.
Here's what you should do as soon as possible:
1) File a complaint with your local police. (Many police departments can take a complaint such as this over the phone.) Be sure to tell them that you're being harassed by an extortionist who is calling you at home and work and threatening you with arrest if you don't pay him money. File a complaint each time they call you. Filing a police report about this is important so your employer knows that you're being harassed by criminals; you may want to ask your employer to help out by filing a police report if and when these crooks call you again at work. (By the way, if this were a real debt collector you can tell him to stop calling you at work and, by law, he must comply; but since these are criminals calling you, I seriously doubt that they would feel compelled to abide by the Fair Debt Collection Practices Act.)
2) File a complaint online with the FBI at
www.tips.fbi.gov Include in your report the same information about how these crooks are constantly calling you at work and home and trying to extort money from you under threat of arrest.
3) File a complaint with the consumer protection division of your state's attorney general. The contact for each state's attorney general is at
www.naag.org4) File a complaint online with the Federal Trade Commission at
https://rn.ftc.gov/pls/dod/wsolcq$.startup?Z_ORG_CODE=PU015) If these crooks call you again, just ignore their threats and tell that you know they're a criminal operation and that you've reported them to FBI and your local police. Once they know that you are aware that they aren't legitimate, I'll bet they cool it for awhile or stop altogether.
Hope this helps. Good luck!