They call our number on a daily basis, but never leave any message. How to stop such calls, any suggestion?

They've called our number for weeks, never leaving a message so just now when they called I answered and they hung up! ARGH!!!

They are calling constantly, everyday, twice a day or more  for weeks now. I do not answer the phone.  They do not leave a message.  Caller ID says Special Olympics.

Called back the 313-768-3264 which had a recording identifying them as Special Olympics and to call 866-544-4880 for further inquires.  I called 866 number, asked why they keep calling and hanging up; rep apologized and started to sell - not answering the questions.  I asked the question again, they still did not answering the question, but the rep after a couple of iterations apologized again and offered to put our number on their do-not-call lists.  Got his name and asked which company he worked for and he did say Special Olympics.  

Will see if this works...

Called the 866 #, and they kindly put my number on the do not call list.  But the gal didn't know anything about the 313 #.  I hope the calls stop.