You've seen the caller ID: Private Caller. This means the person has a number-blocking service through the telephone company, or the person (prior to dialling your number) entered a code (such as *67) that blocks the dialer's number. Because the phone company has blocked this caller's information, it's more difficult to trace a name associated with a Private Caller caller ID than names associated with other masked caller IDs such as "Unknown" or "Restricted." But it's not impossible.
Dial 69. In most of North America, 69 redials the last number that called your phone (contact your local carrier to confirm this code and if there are charges for its use). If, after dialling *69, someone answers the phone, explain you received a call from their number and would like to know who called you. Or, if you hear a recording that identifies a name or a phone number, write that information down.
If you retrieved a name from the recording, conduct a reverse name search for more information on this person. The more unique the name, the easier it is to trace. One free reverse name search is to simply enter the name and region (for example, "Beauchamp Smalltown Arizona") in your Google browser, press Enter, and view the results. Another free search is to enter the name and region in a people search database such as Pipl (pipl.com).
If you retrieved a number from the recording, conduct a reverse number search to find more information (such as a name and location) associated with this number. One free reverse number search is through Google (type the number in the format "333-444-5555" in the Google browser, press Enter, and view the results).
Contact your local carrier and request anonymous call blocking (people calling from blocked numbers will hear a recording informing them to unblock their number if they want the call to go through). This won't unblock past Private Caller names and phone numbers, only future ones. Note: Anonymous call blocking won't stop masked caller IDs made from pay phones, calling cards, and prepaid cell phones.
If you are receiving threatening or abusive calls from a Private Caller ID, contact the business office of your local phone company. Depending on their policies, they may take action (for example, connecting you with their "annoyance desk" or setting you up with a call trace), or they may request you file a formal complaint with local law enforcement. In most jurisdictions, the phone company, as well as law enforcement, will not release personal information (such as a name or address) associated with the Private Caller to you.
To decipher names and numbers associated with masked caller IDs other than Private Caller, use TrapCall (trapcall.com).