I got my first call from them today, and never answer the phone anymore when I don't recognize the caller. I did recognize the VOIP designation, however, which means the call could have originated from anywhere.
Try. Calling the number back after hours and see if get a company name or anything that might ID it. I have done this a few times and found out twice that it was a charity (both breast cancer research).
Left a message on my answering machine that I had won a promotional wireless alarm system. For a (small) monthly fee. Which means I didn't win anything if I have to pay.