"Aggressive, convincing, and threatening"--that's how Iowa's Attorney General's Office describes a rash of phone calls being made by debt collection scam companies.
The caller starts by saying they are collecting some debt you owe to another company. Sometimes they can give you info about the debt, sometimes not. Then they start to get rude and pushy. They claim you'll be getting legal papers. They say you're committing fraud if you don't pay them. Some even threaten that the sheriff is coming to your work or your house with a warrant if you don't pay right now.
It's a scam, plain and simple. No legitimate debt collector will threaten you with jail or arrest. Very few would even call to tell you that a lawsuit was underway. The more outrageous the caller, the better the odds they're a scam.
Do yourself a favor--hang up. Screen your calls. Don't return voicemails that sound like one of these scams. The more you show you're interested/worried/concerned, the more they see you as an easy target.