I have been getting numerous calls from these people (888-278-6801, 877-866-8300, and other numbers) for the past several months. I'm sure I don't owe anyone so I did some research on them. The BBB gives them a rare "F" rating, and there are complaints about them on numerous web sites. More telling, though, is that I checked the local court filings (here in a major city) and found NO lawsuits or judgments by either American Agencies or ACA Receivables. I also checked my credit report and confirmed that there were no adverse notices or any unpaid debts.
The conclusion I draw is that they are a paper tiger. Since the debts they are trying to collect are not legally recognizable, the only weapon they have in their arsenal is phone harassment. To answer the previous person's question, you should definitely NOT return their phone calls or give them ANY personal info (even confirming your name) -- this is playing right into their hands.
If the calls really bother you then you might try buying a phone blocking service or contacting an attorney. Calls that use an auto-dialer or pre-recorded message (like "Please wait for the next agent") are illegal unless used to collect an ACTUAL debt, and you can win damages of $500 to $1500 PER CALL. I haven't sued them (yet) because I suspect it would be difficult to collect any judgment against them. They would probably just change their name and conceal any assets or move them offshore.