Callers who don't want their phone numbers to show up on caller ID can easily block the display of their numbers on individual outgoing calls by dialing *67 before dialing the call. On most carriers, this code blocks the display of the incoming call's number on caller ID. Instead, the incoming call will display as "Private," "Unavailable," "Blocked," or a similar message, depending on the receiver's service. This technique only blocks caller ID for the call made immediately after the code is dialed; all subsequent calls will revert to normal caller ID settings.
Private or Blocked Numbers
Some carriers also offer customers the ability to block caller ID for all calls made from a particular line. In this case, all outgoing calls made from that line will display as "private" or "restricted" on receivers' phones by default, although the caller has the option to suspend caller ID blocking on a per-call basis.
Revealing and Tracing Numbers
Some smart phone apps and offer the ability to reveal private and blocked numbers. These services reveal blocked by routing rejected blocked calls through the service and then returning the blocked number to the user's phone.
Phone carriers also offer the ability to trace calls by dialing *57 after receiving an unwanted call. This code flags the call as unwanted and records the number in a database maintained by the carrier. It does not, however, return the number to you, and there is a per-call charge for the service.
Screening Private Numbers
Some carriers allow customers to automatically reject any incoming call from a private or blocked number. This feature is typically activated by dialing a code and remains in effect until the customer dials another code to deactivate it. When the feature is active, the call will not go through to your phone until the caller unblocks caller ID and allows his number to be displayed on your phone.
Smart phones themselves also offer limited options for screening blocked numbers. These features allow you to either block or assign a special ringtone to incoming calls from all numbers except those on a list of approved callers. These features will block or screen calls from private numbers, but they will also do the same to any number outside of those on the approved list, whether the incoming number is blocked or not.