CALLER ID

Caller ID, known also as Caller Identification (CID), or more properly Calling Number Identification (CNID), is a telephone service, available on POTS (plain old telephone service) lines, that transmits a caller's number to the called party's telephone equipment during the ringing signal, or when the call is being set up but before the call is answered. Where available, caller ID can also provide a name associated with the calling telephone number, in some cases for a higher fee. The information made available to the called party may be made visible on a telephone's own display or on a separate attached device.

Caller ID may be used to track down or limit the impact of prank calls, telemarketers, and other intrusions. However, it can also impede communication by enabling users to become evasive. The concept behind caller ID is the value of informed consent; however, it also poses problems for personal privacy. Another problem is that the possibility of caller ID spoofing makes it unreliable. However, CNID and ANI are not the same thing. Caller ID is made up of two separate pieces of information: the calling number and the billing (or subscriber) name where available. When an originating phone switch sends out a phone number as caller ID, the telephone company receiving the call is responsible for looking up the name of the subscriber in a database. It is for this reason that mobile phone callers appear as WIRELESS CALLER, or the location where the phone number is registered (these vary based on which company owns the block of numbers, not the provider to which a number may have been ported). Additionally, nothing ensures that the number sent by a switch is the actual number where the call originated; telephone switch initiating the call may send any digit string desired as caller ID. As such, the telephone switch and, therefore, the operating entity must also be trusted to provide secure authentication.

The displayed caller ID also depends on the equipment originating the call.If the call originates on a POTS line (a standard loop start line) caller ID is provided by the service provider's local switch. Since the network does not connect the caller to the callee until the phone is answered generally the caller ID signal cannot be altered by the caller. Most service providers however, allow the caller to block caller ID presentation through the vertical service code *67.

A call placed behind a private branch exchange (PBX) has more options. In the typical telephony environment a PBX connects to the local service provider through PRI trunks. Generally, although not absolutely, the service provider simply passes whatever calling line ID appears on those PRI access trunks transparently across the Public Switched Telephone Network (PSTN). This opens up the opportunity for the PBX administrator to program whatever number they choose in their external phone number fields.