A phone call, email or text message from a supposed debt collector is not legally sufficient evidence of debt.

The Fair Debt Collection Practices Act (FDCPA) includes statements such as this:

* Every collector must send you a written “validation notice” telling you how much money you owe within five days after they first contact you. This notice also must include the name of the creditor to whom you owe the money, and how to proceed if you don’t think you owe the money.   ["Written" means snailmailed ... not emailed or texted]

Source:
https://www.consumer.ftc.gov/articles/0149-debt-collection

Therefore, if that caller wants you to pay immediately, before you get the written validation notice via snailmail, or if the caller wants payment via green dot, Western Union, or other untraceable means, then the call is definitely not legitimate.

IF you wish to communicate with the supposed collector / creditor, visit  the following page, for example "Action Letters" to snailmail:
http://www.consumerfinance.gov/blog/debtcollection/

If you KNOW you owe no money, or believe the call was from a fake debt collector, please take the time to read this:
http://www.consumer.ftc.gov/articles/0258-fake-debt-collectors