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For the protein involved in the synthesis of major histocompatibility complex II, see CLIP .

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.

Calling-line identification

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 Canadamarker (though customers often call it 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 Statesmarker.

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.

Some IP phone services (ITSPs, or Internet Telephony Service Providers) support PSTN gateway installations throughout the world. These gateways egress calls to the local calling area, thus avoiding long distance toll charges. ITSPs also allow a local user to have a number located in a "foreign" exchange; the New York caller could have a Los Angeles number, for example. When that user places a call the calling line ID would be that of a Los Angeles number although they are actually located in New York. This allows a call return without having to incur long distance charges.

With cellphones the biggest issue appears to be in the passing of calling line ID information through the network. Cell phone companies must support interconnecting trunks to a significant number of Wireline and PSTN access carriers. In order to save money it appears that many cell phone carriers do not purchase the North American feature Group D or PRI trunks required to pass calling line ID information across the network.


In 1968, Theodore George “Ted” Paraskevakos, while working in Athensmarker, Greecemarker as a communications engineer for SITA, began developing a system to automatically identify a telephone caller to a call recipient. After several attempts and experiments, he developed the method in which the caller's number is transmitted to the called receiver's device. This method was the basis for modern-day Caller ID technology.

From 1969 through 1975, Paraskevakos was issued 20 separate patents related to automatic telephone line identification; they are listed below.

Country Patent number Issue date Description
Greece 40176 May 28, 1969 Apparatus for the decoding and display of the identification of calling telephone devices in the receiving device
Greece 37541 June 19, 1969 Apparatus for the decoding and display of the identification of calling telephone devices in the receiving device
Greece 37733 June 27, 1969 Method of automatically transferring by electrical pulses the identification of a calling device and automatic display in the receiving device in automatic urban and long distance telephony
Greece 38280 October 16, 1969 Method of automatically transferring by electrical pulses the identification of a calling device and automatic display in the receiving device in automatic urban and long distance telephony
Greece 39092 January 26, 1970 Method of automatically transferring by electrical pulses the identification of a calling device and automatic display in the receiving device in automatic urban and long distance telephony
Greece 42452 September 8, 1970 Apparatus for the decoding and display of the identification of calling telephone devices in the receiving device
France 2.133.267 April 15, 1971 Procédé et appareil pour enregistrer le numéro du poste téléphonique dont un appel provient
Japan 22983/1971 May 17, 1971 Improved apparatus for generating and transmitting digital information
France 2.152.356 September 7, 1971 Pulse train generating and selection apparatus
Greece 43263 September 10, 1971 Method of automatically transferring by electrical pulses the identification of a calling device and automatic display in the receiving device in automatic urban and long distance telephony
Greece 43999 February 10, 1972 Apparatus and method for automatically displaying the identification of a calling device in the receiving device
South Africa 71/3894 May 3, 1972 Improved apparatus for generating and transmitting digital information
Italy 935035 December 1, 1972 Apparatus perfezionato per generare e trasmettere informazioni numerche
United States 3,727,003 April 10, 1973 Decoding and display apparatus for Groups of Pulse Trains
Canada 938363 December 11, 1973 Decoding and display apparatus
France 7132207 January 4, 1974 Appareil perfectionné de production et de transmission ď information numérique
United States 3,812,296 May 21, 1974 Apparatus for generating and transmitting digital information
Great Britain 1362411 December 4, 1974 Apparatus for decoding and display of digital information
Great Britain 1362412 December 4, 1974 Pulse Train generating and selection apparatus
Australia 458,841 July 24, 1975 Telephone system

Since Paraskevakos' patents significantly predated all other similar patents, they appear as prior art in later U.S. Patents issued to Kazuo Hashimoto and Carolyn A. Doughty.
The first caller identification receiver

In 1971, Paraskevakos, working with Boeing in Huntsville, Alabamamarker, constructed and reduced to practice a transmitter and receiver, representing the world's first prototypes of caller identification devices. They were installed at Peoples' Telephone Company in Leesburg, Alabama and were demonstrated to several telephone companies with great success. These original and historic working models are still in the possession of Paraskevakos.

In the patents related to these devices, Paraskevakos also proposed to send alphanumeric information to the receiving apparatus, such as the caller's name, and also to make feasible banking by telephone. He also proposed to identify the calling telephone by special code (e.g., "PF" for public phone, "HO" for home phone, "OF" for office phone, "PL" for police).

In May 1976, Kazuo Hashimoto, a prolific Japanese inventor with over 1000 patents worldwide, first built a prototype of a caller ID display device that could receive caller ID information. His work on caller ID devices and early prototypes was received in the Smithsonian Institutionmarker, National Museum of American Historymarker in 2000. U.S. patent 4,242,539, filed originally on May 8, 1976, and a resulting patent re-examined at the patent office by AT&T, was successfully licensed to most of the major telecommunications and computer companies in the world.

Initially, the operating telephone companies wanted to have the caller ID function performed by the central office as a voice announcement and charged on a per-call basis. John Harris, an employee of Northern Telecom's telephone set manufacturing division in Londonmarker, Ontariomarker promoted the idea of having caller ID as a telephone set display. The telephone was coded ECCS for Enhanced Custom Calling Services. A video of his prototype was used to leverage the feature from the central office to the telephone set.

Early in 1977 Nélio Nicolai, a Brazilian inventor, created a machine capable of identifying and displaying the caller ID, he named it BINA (B identifies number of A). He patented the invention, but lack of support from the Brazilian patent-issuer authority INPI made him pursue the judicial system to effectively collect the royalties from his invention (the lawsuit is still running ). He received many proposals to drop his wish for a full patent recognition, but did not accept any. The first commercially available BINAs appeared in 1982 in the Brazilian capital, Brasíliamarker. In 1996, Nélio received the WIPO (World Intellectual Property Organization) award for his invention. There is an ongoing debate on how Bell Canada approached Nélio and its former employer ( Telebrasília) in the mid-80s asking for technical papers and prototypes (Nélio travelled to Canada). Experimental use of the system happened at the Canadian city of Peterboroughmarker, Ontariomarker. Talks ceased and surprisingly, in 1986, Bell Canada announced such tests to the general public, with following commercialization starting in 1988. Nélio says that no royalties were ever collected.

The first market trial for Caller ID and other "Custom Local Area Signaling Services" was held in early 1984 by Bell Atlantic in Harrisburg, Pennsylvania. The service was later trialed by BellSouth as one of the "TouchStar" services on July 7, 1984 in Orlando, Floridamarker. The marketing departments in both telephone companies choose to use the name "Caller ID". Planning for both trials was initiated by a team in Bell Laboratoriesmarker, AT&T, and Western Electric before the Bell System divestiture, with the participation of Bell of Pennsylvania and Southern Bell. The purpose of these trials was to assess the revenue potential of services that depend on deployment of the common channel signaling network needed to transmit the calling number between originating and terminating central offices. Trial results were analyzed by Bellcore members of the original team.

In 1987, Bell Atlantic (now Verizon Communications) conducted another market trial in Hudson Countymarker, New Jerseymarker, which was followed by limited deployment. BellSouth was the first company to deploy caller ID in December 1988 in Memphismarker, Tennesseemarker, with a full deployment to its nine-state region over the next four years. Bell Atlantic was the second local telephone company to deploy Caller ID in New Jersey's Hudson County. US West Communications (now Qwest) was the third local telephone company to offer caller ID service in 1989.

Type II Caller ID

In 1995, Bellcore released another type of modulation similar to Bell 202 in which it became possible to transmit caller ID information and even provide call-disposition options while the user was already on the telephone. This service became known in some markets as call waiting ID, or (when it was combined with call-disposition options), Call Waiting Deluxe; it is technically referred to as Analog Display Services Interface. "Call Waiting Deluxe" is the Bellcore (now Telcordia Technologies) term for Type II caller ID with Disposition Options.

This CLASS-based POTS-telephone calling feature works by combining the services of call waiting with caller ID but also introduces an "options" feature that, in conjunction with certain screen-based telephones, or other capable equipment, gives a telephone user the option to

  • Switch: Place the current call on hold to take the second call (not a new feature)
  • Hang-up: Disconnect the current call and take the second call
  • Please Hold: Send the caller either a custom or telephone-company-generated voice message asking the caller to hold
  • Forward to Voice Mail: Send the incoming caller to the recipient’s voice mail service.
  • Join: Add the incoming caller to the existing conversation.


In the United States, caller ID information is sent to the called party by the telephone switch as an analog data stream (similar to data passed between two modems), using Bell 202 modulation between the first and second rings, while the telephone unit is still on hook. If the telephone call is answered too quickly after the first ring, caller ID information will not be transmitted to the recipient. There are two types of caller ID, number only and name+number. Number only caller ID is called Single Data Message Format (SDMF), which provides the caller's telephone number, the date and time of the call. Name+number caller ID is called Multiple Data Message Format (MDMF), which in addition to the information provided by SDMF format, can also provide the directory listed name for the particular number. Caller ID readers which are compatible with MDMF can also read the simpler SDMF format, but an SDMF caller ID reader will not recognize an MDMF data stream, and will act as if there is no caller ID information present, e.g. as if the line is not equipped for caller ID.

Instead of sending the caller ID in between the first and second ring, some systems use a "line reversal" to announce the caller ID, or caller ID signals are simply sent without any announcement. Instead of Bell 202, the European alternative V.23 is sometimes used, (without the 75 baud reverse channel) or the data is sent using DTMF signalling.

In general, CID as transmitted from the origin of the call is only the calling party's 10-digit phone number. The calling party name is added by the consumer's terminating central office if the consumer has subscribed to that service. Calling name delivery is not automatic. An SS7 (or Signaling System 7) TCAP query must be launched by the called party's central office to retrieve the information for Calling Name delivery to the caller ID equipment at the consumer's location. Canadian systems automatically send the calling name with the call set up and routing information at the time of the call.

To look up the name associated with a phone number, the carrier in some instances has to access that information from a third-party database and some database providers charge a small fee for each access to such databases. To avoid such charges, some carriers will report the name as "unavailable", or will report the name as "(city), (state)" based on the phone number, particularly for wireless callers. For 800 numbers, they may report a string such as TOLLFREE NUMBER if the name is not available in a database.



Telemarketing organizations often spoof caller ID. In some instances, this is done to provide a "central number" for consumers to call back, such as an 800 number, rather than having consumers call back the outbound call center where the call actually originated. However, some telemarketers block or fraudulently spoof caller ID to prevent being traced. It is against United States federal law for telemarketers to block or to send false caller ID. Individuals can bring civil suits and the FCC can fine companies or individuals that are illegally spoofing or blocking their caller ID.

Some telemarketers have used caller ID itself for marketing, such as by using an 800 number and the text string "FREE MONEY" or "FREE PLANE TICKETS" as the name to be displayed on the caller ID.


Some Internet service providers (ISPs) providing dial-up access require the customer to use CNID to prevent abuse of the account by unauthorized callers. Some systems with dial-up access can be programmed only to accept calls with specific caller ID strings.

Mobile providers

Most mobile phone providers used the caller ID to automatically connect to voice mail when a call to the voice mail number was made from the associated mobile phone number, bypassing the need to enter a password. While this was convenient for many users, because of spoofing, this practice has been replaced by more secure authentication by many carriers.


Caller identification between countries is often not transmitted, the display indicating "UNAVAILABLE" or "INTERNATIONAL" but is becoming more widely available including the country calling code. This is usually the case with mobile phones and international roaming. However, the overseas number may be compressed into a "domestic" format and thus possibly not be recognizable: e.g. a US number 1 646 555 1212 may be displayed in the UK as 0646 555 1212, instead of 001 646 555 1212 (or as +1 646 555 1212, where the "+" represents the access code to dial international numbers).

Some callers have situations in which revealing the number being called from would invade their privacy or cause other severe problems. Calls from women's shelters often have outgoing caller ID blocked.

Caller ID may not give the caller's information if the caller is using a discount or travel calling program or a voice over IP phone, because those systems use intermediate telephone numbers in the country being called.

Some long distance telephone cards, especially those using VOIP to carry calls, mask caller ID information. These calls are commonly displayed as "Out of Area".

Legal issues

United States

In the United States, telemarketers are required to transmit caller ID. This requirement went into effect on January 29, 2004. Courts have ruled that caller ID is admissible. Providers are required by FCC rules to offer "per-call" blocking of caller ID to their customers. Legislation pending in the United States in would make it illegal to "spoof" caller ID for fraudulent purposes.

Getting around caller ID


Blocking is the common term for preventing the display of a calling number.

Telecommunications regulators vary in their requirements for the use and effectiveness of assorted technologies to prevent numbers from being displayed. Generally, unlisted numbers are always blocked. Non-published and regular listed numbers are not usually blocked. But there is varying treatment for the determination of call display blocking because of many factors. If desired, customers should inquire carefully to make sure their number will not be displayed.

In some locations in the United States, regulators allow (or require) blocking to be automatic, transparent to the caller.

Where blocking is applied on a call-by-call basis (at the time a call is made), subscribers can block—prevent their numbers from being displayed—by dial a special code before making a call. In North America and some other regions, the code is *67, while the United Kingdommarker and Irelandmarker use 141. This special code does not block the information from companies using call capture technology. This means that equipment with caller ID will simply display the word "PRIVATE" or "WITHHELD". When CNID is blocked at the caller's request, the number is actually transmitted through the entire telephone network, with the "presentation withheld" flag set; the destination CO is expected to honor this flag, but sometimes does not—especially when the destination phone number is served by an ISDN primary rate interface.

Alternatively, in cases where caller ID is being blocked automatically, it can only be released on a call-by-call basis by dialing a special code (*82 in North America; 1470 in the UK). See "Disabling", below.

Similarly, some countries offer anonymous caller rejection, which rejects all calls when the subscriber's number is blocked. Some telephone companies protect their clients from receiving blocked information by routing anonymous calls to a service, where the caller is required to announce him or herself. The service then asks the called party if they want 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), or by using general ANI services.

These features setup a cat-and-mouse game type of situation where subscribers must purchase additional services to cancel out other services.


Adding to the cat-and-mouse game of Caller ID is the fact that even if you block your number, it can still be trapped as it is still sent through the telephone company network. In the U.S. the FCC requires the number to be transmitted to toll-free numbers regardless of whether the number is blocked. Taking advantage of another telephone company feature, call forwarding, some telephone customers have call forwarded their local phone numbers to a toll-free number. When the local number is called by a customer who is blocking their caller ID, the local number is forwarded to a toll-free number, where the number of the calling party is passed on to and the owner of the local number can receive the information. Taking advantage of this, services have popped up such as the popular TrapCall for US based cellphone users. TrapCall works by conditionally forwarding only your unanswered and rejected calls to the service. When a customer rejects an incoming blocked call it is routed to TrapCall, who in turn traps the number and reroutes the call back to the cellphone user with the Caller ID displayed giving the customer the option to answer the call or reject it once more to send it back to TrapCall where the second call will be sent to voicemail. Also some shady CLEC's ignore the privacy bit altogether on their phone numbers that they offer customers.


Caller ID is a simple string of data that can be read and generated very simply by small computer programs and even some inexpensive devices. Although the caller ID information is transmitted in between the first and second rings, before the phone call is connected, it is possible to mislead the phone company as to the origin of the call, thus defeating caller ID. Although misleading the phone company by generating Automatic Number Identification (ANI) code or by other electronic means is very complex, simpler means do exist. Using services like Vonage or Skype, calling from a pay phone, or using a calling card are easy means to fool Caller ID. Services like Operator One and Rebtel use Caller ID replacement to get around international call charges for missed calls that are returned by the dialed up party.


In North America, the code to disable caller ID is *67. In Argentina, the code is *31# (landlines) or *31* (most cell phone companies). In the United Kingdom and Ireland, 141 is the equivalent code. Australia uses 1831. New Zealand uses 0197, in NZ you can also request Telecom to permanently disable your caller ID. Hong Kong uses 133. Israel uses *43. In Denmark and Switzerland *31* is used to hide the one's called ID; the same code is used to reveal your caller ID if you have your number set-up as a permanent disable. Other countries and networks may vary. On GSM mobile networks, callers may dial #31# before the number they wish to call to disable it.


When the caller has caller ID disabled by default, there is a code to enable caller ID, which is operator dependent. In North America it is generally *82 (*UB, Un Block). Landlines in Ireland can should dial 142 before entering the number to be called. Landlines in the United Kingdom are able to unblock their caller ID by pre-fixing the dialled number with 1470. On GSM mobile networks, callers may dial *31# before the number they wish to call to enable caller ID.


  • Calling ID (vs. Caller ID) is the identification of whom you are calling, or connecting to, as opposed to caller ID identifying who calls you. Some Centrex telephone systems offer this feature. Similarly, when one Skype user calls another Skype user, the caller can see the other party's details and even an image or photograph they have chosen to represent their identity.
  • The inverse feature, giving the number originally dialled, is known as Direct Inward Dialing, Direct Dialing Inward, or Dialed Number Identification System. This tells the PBX where to route an incoming call, when there are more internal lines with external phone numbers than there are actual incoming lines in a large company or other organisation.
  • Not all types of caller identification use 202-type modulation, nor do all systems send the information between the first and second ring, e.g., British Telecom sends the signal before the first ring, after a polarity reversal in the line. (Because of this most caller ID software is not compatible with BT even if the modem is) As a result, not all caller ID devices are compatible from country to country or in the same country, even though the basic phone system is the same. Some providers use FSK, others use the DTMF protocol.
  • This is not to be confused with Microsoft Caller-ID, a patented e-mail authentication technology. For the history of this now-dead proposal, see MARID.
  • Caller ID as now implemented is not as originally represented to the public, as to the Call Blocking feature. Regardless of blocking status, your Caller ID is always transmitted with each call and never actually blocked (see above). Much like the Social Security Numbers' supposed limited use by law, the caller ID is everywhere and unavoidable.


  1. formerly known as Société Internationale de Télécommunications Aéronautiques
  2. Patent # 4,242,539/12-30-1980
  3. Patent # 4,551,581/11-5-1985 and Patent # 4,582,956/4-15-1986; (both assigned to AT&T Bell Laboratories)
  4. PhoneTel Patent Services :: History : Hashimoto
  5. PhoneTel Collection, 1954-1994
  6. Fight heats up over patents on Caller ID. (Kazuo Hashimoto) - Communications News - HighBeam Research
  7. 47 CFR 64.1601
  8. 18 FCC Rcd 14014 (FCC, July 3, 2003) at para. 173 et seq.
  9. 47 C.F.R. § 64.1601(e).
  10. State v. Schuette, 273 Kan. 59, 44 P.3d 459 (Kansas 2002)
  11. GSM Features

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