Just a junk debt buyer, however there is apparently some evidence they are connected to NCO Financial, and possibly other junk-brand names, so take care they don't double-team or otherwise play you.
Their contact information is as follows:
Merchants' Credit Guide Co.
223 West Jackson Blvd, Suite 900
Chicago, IL 60606-6912
Consumers are told to call: 888-249-4134
and other multiple rotary line numbers.
Their callers use a host of made-up names, so don't pay any attention to them; just answer the phone and indicate that you are required to advise them this call is being recorded in the case of future litigation. They will either immediately hang up or suddenly become professional and courteous.
Ask them what you want to know; if the caller is too incompetent to answer your questions, or they continue to make harassing calls, then mail them a certified 'stop-calling' letter and save your signature receipt.
If there is any real basis for collection, they will either write you a letter to offer a deal or sell the debt to someone even lower on the food chain, if that's possible.