Caller identification, caller ID or CID, is a now routine technology that allows someone receiving a telephone call to see the name and telephone number of the individual or business attempting to make contact with him or her. The individual being called then has the option of accepting the telephone call at that moment, returning the telephone call at another time or simply ignoring the attempted contact and any message left. The technology is not foolproof. The US's Federal Communications Commission (FCC) warns of con artists who manipulate the technology to reflect erroneous names and telephone numbers in a process known as spoofing. In addition, this technology is not a permanent installation and a user can reverse caller ID by codes, telephone settings or contacting his telephone company or cell phone carrier.
A long-term method to reverse caller ID is to contact the telephone company or cell phone carrier associated with the number. Depending upon the company, obtaining an anonymous "Unknown Caller" identity may incur an additional monthly cost. This method will remain in effect for all outgoing telephone calls until the service is discontinued by contacting the company; however, some companies allow a caller to temporarily show the full caller ID information on a call-by-call basis by dialing a code prior to each telephone number. This feature is known as per line blocking.
Some brands of mobile phones, particularly smartphones, allow their users to reverse caller ID in their system settings menu. This anonymity is usually free of charge as part of the telephone options; however, charges may apply with some mobile carriers. The setting will remain in effect until the telephone owner changes the system settings to again display his name and number.
Regardless of whether a mobile phone or a landline telephone number is involved, a caller can reverse caller ID on a per call basis. This is put into effect by entering or dialing the code *67. There is no cost associated with this service and the code must be reentered prior to each outgoing telephone call to continue the caller's anonymity.
When calls are made to certain numbers, reverse caller ID may not work. For example, this service will not work if an emergency number like 911, a police or a poison control telephone number is dialed. Many toll-free telephone numbers will also continue to show the name and telephone number of the caller despite any call blocks in place.