Unscrupulous people buy up old consumer lists and old debts and then call people and harass and bully them into thinking they owe money that they don't. In this way they scam people out of their money. If you are sure you've never took out this loan, you're probably right and need to dispute the debt.
You have substantial rights under a Federal act called the Fair Debt Collection Practices Act. This kind of harassment and coercion is unlawful under state and Federal law. You don't have to buckle to them and you certainly shouldn't give them a credit card number or agree to pay them unless/until they can prove to you that you really owe this debt.
You can ask them when next they call for a letter in writing validating that debt, as is your right, and whether they give it to you or not, you can dispute the debt if you don't believe that you owe it. You're entitled to do that under the FDCPA.
Once you formally dispute the debt, which you should do in writing, they MUST leave you alone and you are free to ignore them. If they continue to bother you, you could report these folks to the Federal Trade Commission as they would be violating the Fair Debt Collection Practices Act and can be fined $1,000 for each time they illegally harass you.
If they are scammers and not even real collection agencies, they are probably not going to be intimidated. However, once you tell them that you're disputing the debt, you can safely ignore them.
In the worst case scenario, if they are real and truly believe you owe this debt, they would have to take you to civil court and you could prove there that the debt does not exist. But it shouldn't come to that.
The Federal Trade Commission gives a wonderful overview of overaggressive and/or fake collection agencies and how to get them off of your back, where to report them, where to learn more, etc.
You can access that here: