How many calls have you been getting and what time of day have these calls been coming in? What have you heard when you answer the phone?
If the calls are coming in at night after 9:00 pm then you have a stalker to worry about, this you have to take to the police and the cell phone company, and if the calls are coming from another cell phone, they can find the person making them. If the calls are coming from a standard landline account, they can find that landline. If the calls are coming over a VOIP setup, that's going to be harder to trace.
If the calls are coming in the daytime before 9:00 pm then it is possible you are getting calls from fraudulent telemarketers. There has been a huge rash of these brats making calls to every possible telephone number trying to sucker people into giving them credit card numbers or other personal non-public identification. The pitches are for car warranties, burial insurance, diabetic medication, senior citizen assistance, credit-card account adjustments, etc. etc. My land line gets 5 of these a day, except on Sunday. My cell phone gets at least two of these a week, but since I have never given the cell number to anyone but relatives I figure it's an unknown number to these pests. I have gotten these calls for years, but the current barrage started in November 2007 and has continued every day since.
These fraudulent calls are made by a "robo-call" or automatic dialing machine, which detects if the call is answered and then starts its prerecorded message. Sometimes the machine does not detect the answer and does not start its prerecorded message, all that is there is silence until you hang up.
As these businesses are fraudulent, they ignore the "Do Not Call List" where you may have already registered your cell number.
How to Handle Unwanted Phone Calls
http://www.ehow.com/how_2368760_handle-unwanted-phone-calls.html
More information:
http://www.mpowercom.com/unwanted_calls.htm
The Federal Trade Commission says:
http://www.fcc.gov/cgb/consumerfacts/tcpa.html