Caller ID

What Does Caller ID Mean?

Caller ID is a telephone feature that displays a caller’s phone number on the recipient’s phone device before the call is answered. The phone number, location and associated billing or subscriber name is shown on the handset’s display or a separate caller ID box attached to the phone.


The CLID feature is particularly useful when screening known, unknown, or unwanted calls. Unwanted calls are effectively deterred with the use of caller ID, including obscene, harassing and threatening phone calls.

Caller ID service, also known as caller identification (CID), calling line identification (CLID), and calling number identification (CNID), is provided by phone companies for analog and digital phone systems, as well as Voice over Internet Protocol (VoIP) applications.

Techopedia Explains Caller ID

To enable caller ID, the phone company equips a phone with a caller ID box. This usually includes a modem to decode data bits, a tiny circuit to detect a ring signal and a simple processor to drive the display. Caller ID data is transmitted between the first and second rings. When a call is answered immediately after the first ring, caller ID may not be available.

Caller ID is one of two types as follows:

  • Number Only: Single Data Message Format (SDMF) is used, where the displayed information includes only the caller’s telephone number and the date and time of the call.
  • Number Plus Name: Multiple Data Message Format (MDMF) is transmitted, and the directory name is added to the displayed information.

Related Terms

Margaret Rouse

Margaret is an award-winning technical writer and teacher known for her ability to explain complex technical subjects to a non-technical business audience. Over the past twenty years, her IT definitions have been published by Que in an encyclopedia of technology terms and cited in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine, and Discovery Magazine. She joined Techopedia in 2011. Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages.