I called the number back and it says its the "automated removal number." You can press a number and get a copy of their "Do Not Call Number Policy." My guess is that they are telemarketers, not bill collectors.
They just called my cell again,,, was talking to a friend and it seems they are getting cell numbers (in our cases) from online gaming applications,, mine was off of myspace when I got free credits while playing a game,,, two other occasions were related to a friend on facebook,, same story,, he signed in and did a free survey to get credits for a game he was playing,,, little did he know his number was given out to every scam artist in the country,,,