To everyone who talks to these vultures over the phone, know that any verbal agreement you make isn't worth with the electrons it's transmitted on over the lines. EVERTHING must be done in writing. As per the Fair Debt Collections Practices Act, once you send a certified, return receipt requested letter to them asking for proof of debt, they have 30 days in which to provide said proof of debt. But you can't wait - send that letter out ASAP. If they do not provide proof of the debt and still harrass you, then you have a strong complaint for the States Attorney General and possibly grounds for a lawsuit.
Most of these collection agencies are barely legal, hack outfits that buy computer printouts from companies. Most of the time, your name is just a line on a print out with little other information than the last known phone number.
Remember - NO COMMUNICATION BY PHONE. If they have you as a legitimate debt holder then they have your mailing address. DO NOT GIVE THEM ANY INFORMATION. If they don't have the proper information then it is most likely a scam.