These guys are running a scam. Basically it is a standard debt collection scam, where they claim you owe something that you do not and threaten you with arrest, etc. They usually claim the debt is for a payday loan because people often think that non-payment on such a loan is the same as check fraud, which it is NOT.
Remember you cannot be arrested for non-payment of a loan. Do NOT pay them anything. Don't even answer the phone or return their calls. Keep all voicemails and make a report to the following organizations:
1. FBI - the FBI is actively pursuing these types of scams and are very interested in tips. Report any calls and threats made by these scammers to the FBI so we can put these guys in prison where they belong.
2. Your state attorney general - they are also interested in putting a stop to these scammers.
3. Level 3 Communications - the guys are using Level 3 communications' network (VoIP) to make threats and to extort money out of people. Send an abuse report to Level 3 for each threat noting the time and date of the call. Offer to forward voicemails. If more people start complaining, Level 3 will cut their service off. They won't be able to harass you if they don't have a phone :)
Again, these guys are scammers, Don't speak with them. Don't give them money or any personal information.