They are a collection agency. Google them. You will see a lot of complaints.
I would not deal with them until they provide you verification of the debt and that they are authorized to collect it.
Did you, in fact, take out a previous payday loan and not pay it back? If so, make them specifically identify the loan and account for the balance due. If not, then ignore them.
I have seen several statements on the web that claim that this is a scam. I do not have any specific knowledge of that fact but I would be wary if I were you especially since they will not give you specific facts or an address, etc.
They are a debt collector. Because of the type of loan, they maintain they are not covered by the FDCPA. Trust me, I have voice mail recordings by other debt collectors claiming they are holding "warrants" or threatening all manner of things which have made me cringe. You may want to try and settle the matter with the lender otherwise, the debt collector will also call everyone you listed for a referrence, and anyone who cross references to you on you credit report.
Good luck to you.
First, is there debt owed? Secondly, if so - obtain proof of there authority to collect the debt. Finally, they should be willing to put all the matter in writing. As to whether this is on the up and up, without more - het documentation.