Caller ID is a standard feature of cell phones, and most of the landline phones are also equipped with it. Using this feature, you can find out the number and name of the person who calls you. However, a caller can prevent his name and number from being revealed on the caller ID by dialing a specific code before making the call. As a result, you will see "unknown" or "withheld" on your caller ID screen. When you receive any such call, you might want to find out who it's from before receiving.
Contact your network carrier and find out if they offer "Anonymous Call Rejection" service. For most network carriers, such as AT&T, Verizon and Sprint, you can activate anonymous call rejection by dialing "*77." Activating the feature will send a voice message to the withheld caller letting him know that the call recipient doesn't accept withheld calls. The voice message will tell the caller to dial a particular number to reveal his number.
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Find out if your network carrier offers the service of "Privacy Director." This feature can intercept all "unidentified," "withheld," "unavailable" or "out of the area" calls before your phone rings. AT&T provides this service. You can subscribe to it for a monthly fee.
Do not answer the call and let it go to your voice mail. The caller might leave his name. Another option is to pick up the phone and politely request the caller to reveal his identity.