Your first instinct is probably correct. I looked at another website, which stated the caller had threatened legal action if their call was not returned. Don't call them back. If they call you again, arm yourself with this information from the CFPB:

How can I verify whether or not a debt collector is legitimate?
Here are a few warning signs that could signal a debt collection scam:

The debt collector threatens you. Legitimate debt collectors probably won't claim that they'll have you arrested or claim that they or their employees are law enforcement officers.

The debt collector refuses to give you information about your debt or is trying to collect a debt you do not recognize. You have certain rights to ask a debt collector to verify the debt. You can use this sample letter to request this information. Ask for an explanation in writing before you pay.

The debt collector refuses to give you a mailing address or phone number.

The debt collector asks you for sensitive personal financial information. You should never provide anyone with your personal financial information unless you are sure they're legitimate.
If you think that a caller may be a fake debt collector:

Ask the caller for his or her name, company, street address, telephone number, and professional license number. Many states require debt collectors to be licensed. Check the information the caller provides you with your state officials, or the state in which the debt collector holds a license. If the caller refuses or is unable to provide you with information about his company, or if you can’t verify the information he provides, do not give money to the caller or company.

Tell the caller that you refuse to discuss any debt until you get a written "validation notice." This notice must include:
The amount of the debt
The name of the creditor you owe
A description of certain rights under the federal Fair Debt Collection Practices Act
If a caller refuses to give you all of this information, consider requesting this information in writing or seeking assistance before paying the debt to make sure the debt, and the company, are valid.

Tip: You can consider sending the collector a letter requesting the information by using one of the CFPB's sample letters. You can also submit a complaint to the CFPB or you can contact your state Attorney General’s office.

Do not give the caller personal financial or other sensitive information. Never give out or confirm personal financial or other sensitive information like your bank account, credit card, or Social Security number unless you know the company or person you are talking with is a real debt collector. Scam artists, like fake debt collectors, can use your information to commit identity theft such as:
Charging your existing credit cards
Opening new credit card or checking accounts
Writing fraudulent checks
Taking out loans in your name

Contact your creditor. If the debt is legitimate – but you think the collector may not be – contact your creditor about the calls. Share the information you have about the suspicious calls and find out who, if anyone, the creditor has authorized to collect the debt.

Report the call. Submit a complaint with the CFPB or get in touch with your state Attorney General's office with information about suspicious callers.

Stop speaking with the caller.If nothing else works and you believe the calls are fraudulent, send a letter demanding that the caller stop contacting you, and keep a copy for your files.

Tip: You can use this sample letter to write a letter demanding the debt collector stop contacting you. By law, real debt collectors must stop calling you if you ask them to in writing.

Tip: If you're having trouble with debt collection, you can submit a complaint with the CFPB online or by calling (855) 411-CFPB (2372).

http://www.consumerfinance.gov/