Type II Caller ID

Caller ID (caller identification, CID), also called calling line identification (CLID), Calling Line Identification (CLI), calling number delivery (CND), calling number identification (CNID), calling line identification presentation (CLIP), or call display, is a telephone service, available in analog and digital telephone systems, including , that transmits a caller's telephone number to the called party's telephone equipment when the call is being set up. The caller ID service may include the transmission of a name associated with the calling telephone number, in a service called CNAM.

The recipient may inspect the information before answering the call on a display in the telephone set, on a separately attached device, or on other digital displays, such as cable television sets when telephone and television service is provided by the same vendor.

In some countries, the terms caller display, calling line identification presentation (CLIP), call capture, or just calling line identity are used; call display is the predominant marketing name used in Canada (although some customers still refer to it colloquially as "caller ID"). The idea of CNID as a service for POTS subscribers originated from automatic number identification (ANI) as a part of toll free number service in the United States.

However, CNID and ANI are not the same thing. ANI was originally a term given to a system that identified the telephone number placing a call, in a non-electronic central office switch. Previous to this, the calling number could not be identified electronically. Caller ID is made up of two separate pieces of information: the calling number and the billing (or subscriber) name where available. When a call is made from a given name, this name can be passed on through a number of different methods. For example, the caller's name may be datafilled in the originating switch, in which case it is sent along with the number. More commonly, a database is accessed by the receiving switch, in order to match the number to a name. If the name does not exist, then the city, State, Province, or other designation may be sent. Some of these databases may be shared among several companies, each paying every time a name is "extracted". 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; the 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.

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