Caller ID

Caller ID (Caller Identity Display or CID) is a telephony network service that transmits the caller's telephone 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. Typically, CID is transmitted digitally using Bell 202 modulation between the first and second rings. Caller ID is also known as Calling Line Identification (CLI) when provided via an ISDN connection to a PABX, while in some countries, the terms Caller Display, Calling Line Identification Presentation (CLIP) or just Calling Line Identity (CLID) are used. CID originated with automatic number identification (ANI) in the U.S. This is not to say that CID and ANI are the same thing. Caller ID is made up of two separate entities: the calling number and the billing (or subscriber) name. When a phone switch sends out a number, the remote telephone company is responsible for looking up the name of the subscriber in a common database. Additionally, nothing ensures that the number sent by a switch is the actual number where the call originated. It is very easy for a telephone switch to send any digit string it requests to the system. For example, if the originating PBX (say, from an office with an ISDN PRI line) sends out 2024561414 as the ANI, then the remote caller ID box will display the White House on the terminal device.

Subscribers can prevent their numbers from being displayed, or "blocked", by dialing a special code before making a call. This means that equipment with Caller ID will simply display the word 'PRIVATE' or 'WITHHELD'. Alternatively, Caller ID can be blocked permanently, and can only be released on a call-by-call basis by dialing a special code. This is the case in some countries with people who choose not to be listed in the local telephone directory. Similarly, some countries have a service known as anonymous caller rejection, meaning that calls made from a line in which the subscriber's number is blocked, will not be accepted. Some telephone companies also route anonymous calls to a service which requires the caller to announce him or her self, and then requires the called party to accept or reject the call. Blocking the number is referred to as Calling Line Identification Restriction (CLIR). Emergency services will most likely be able to show the restricted number using a service called Calling Line Identification Restriction Override (CLIRO).

Generally, Caller ID between different countries is not transmitted, meaning that the equipment with Caller ID will simply display either 'UNAVAILABLE' or 'INTERNATIONAL'. However, Caller ID between countries is becoming more widely available, meaning that a telephone number in another country will be displayed, complete with the country calling code.

Telemarketing organisations often block the display of their calling numbers. Some states and countries require telemarketers to display a contact number that can accept complaints, as the individual caller numbers may not be able to be called.

Many Internet service providers (ISPs) providing dial-up access require the customer to use CLI to prevent abuse of the account by unauthorised callers.

Type II caller ID also works with call waiting.