Do not pay them because this is highly likely a scam.
Legit collectors per section 809 of the FDCPA http://www.ftc.gov/bcp/edu/pubs/consumer/credit/cre27.pdf *must* tell you at initial contact, or send you paperwork within 5 days of initial contact, the following:
1. $amount of debt,
2. name of the current creditor (which may differ from the original creditor who might have sold your debt),
3. that debt will be assumed to be valid unless you dispute in writing within 30 days,
4. that if you do dispute in writing within 30 days, the collector must mail you written verification of the debt,
5. that if you ask the collector in writing within 30 days, the collector must send you the name and address of the original creditor if it's different from the current creditor.
If you don't get the 5 disclosures, orally or in writing (you do not have to ask for it, the FDCPA requires it), treat the collector like you would any SCAMMER. Scammers are using bought/hacked loan application data to extort payments for fictitious debts from victims and victims' families and friends.
The FBI warns about payday loan extortion scams here http://www.fbi.gov/news/pressrel/press-releases/paydayloanscam_120710 
The BBB has issued several alerts on this scam, just google "bbb.org alert phony debt collector"
Report to the Internet Crime Complaint Center http://www.ic3.gov/complaint/default.aspx , the Federal Communications Commission http://esupport.fcc.gov/complaints.htm , the Federal Trade Commission https://www.ftccomplaintassistant.gov/ and your state AG, as appropriate.