Hope it's not "US National Bank" calling you. That sounds like their MO: calling on pay-day loans and refusing to identify who they are. Do these idiots just expect people to start handing over money to someone that just calls out of the blue and will not identify who they are or what company they are with!
Did whoever you talked to have Indian accents?
Whoever they are, do not give them any money if they refuse to validate the debt in writing and do not give them any banking information whatsoever.