Caller ID (caller identification, CID), also called calling line identification (CLID) or calling number identification (CNID) or Calling Line Identification Presentation (CLIP), is a telephone service, available in analog and digital phone systems and most Voice over Internet Protocol (VoIP) applications, 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. The information made available to the called party may be displayed on a telephone's display or on a separately attached device.
Caller ID may be used to display a caller's telephone number (and, in association with a database, name) on a called user's telephone. This works in most countries; although systems are incompatible, each country will have appropriate equipment. A modem can pass CLID information to a computer for purposes of call logging or blocking, but this can be problematic as modems in different countries have different systems, causing hardware or software incompatibilities.
Caller ID may be used to track down or limit the impact of telemarketers, prank calls, 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.
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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 use the phrase 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. 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 Primary Rate Interface (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 calling 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 Athens, Greece as a communications engineer for SITA,[1] 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 | Apparato perfezionato per generare e trasmettere informazioni numeriche |
| 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[2] and Carolyn A. Doughty.[3]
In 1971, Paraskevakos, working with Boeing in Huntsville, Alabama, 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,[4] 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 Institution, National Museum of American History in 2000.[5] 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.[6]
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 London, Ontario 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 or Binary Identifies Number Address). 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)[7]. 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, Brasília. 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 Peterborough, Ontario. 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 conducted by BellSouth as one of the "TouchStar" services on July 7, 1984 in Orlando, Florida. The Lines of Business (marketing) department in BellSouth Services named the service "Caller ID". The other Regional Bell Operating Companies later adopted the name and eventually became the generally accepted name in the US. Planning for the trial was initiated by a team in Bell Laboratories, AT&T, and Western Electric before the Bell System divestiture, with the participation of 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 County, New Jersey, which was followed by limited deployment. BellSouth was the first company to deploy caller ID in December 1988 in Memphis, Tennessee, 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.
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
In the United States, caller ID information is sent to the called party by the telephone switch as an analogue 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 Signalling System 7) TCAP query may 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 if the caller's name has not already been associated with the calling party's line at the originating central office. Canadian systems using CCS7 automatically (but not in all cases) 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.[8] Individuals can bring civil suits and the Federal Communications Commission (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.
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, instead with the display indicating "UNAVAILABLE" or "INTERNATIONAL", but it 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".
Different countries often use different standards for transmitting caller ID. As a result, phones purchased in one country may not be compatible with the caller ID standard use when the phone is used in a different country.[9] For example, US uses Bellcore FSK whereas Taiwan uses ETSI FSK so a phone purchased in the US will not understand Taiwan's caller ID standard. There are even cases where individual state/provinces will use different protocols within a country. There do exist, however, caller ID converters that will translate from one standard to another.[10] Below is a list of countries and the caller ID standard used:
| Country | Caller ID standard |
|---|---|
| United States | Bellcore FSK |
| Canada | Bellcore FSK |
| China | Bellcore FSK |
| Hong Kong | Bellcore FSK |
| Ireland | ETSI FSK V23 (ETS 300 659-1) Ring Pulse Alert Signalling. Data sent after first short ring. |
| Taiwan | DTMF / ETSI FSK |
| United Kingdom | SIN227 (V23 FSK before first ring) |
| Japan | V23 FSK / DTMF |
| Spain | ETSI FSK |
| Brazil | Bellcore FSK (GVT) / V23 FSK / DTMF |
Telephone equipment usually displays CLID information with no difficulty. Modems are notoriously problematical; very few modems support the British Telecom standard in hardware; drivers for those that do often have errors that prevent CLID information from being recognised.[11] Other UK telephone companies use slight variations on the Bellcore standard, and CLID support is "hit and miss".[12]
In the United States, telemarketers are required to transmit caller ID.[13] This requirement went into effect on January 29, 2004.[14] Courts have ruled that caller ID is admissible.[15] Providers are required by FCC rules to offer "per-call" blocking of caller ID to their customers. Legislation in the United States in 2007[update] would make it illegal to "spoof" caller ID for fraudulent purposes. See Caller ID Spoofing
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 dialing a special code before making a call. In North America and some other regions, the code is *67, while the United Kingdom and Ireland 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 "Enabling", 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 create 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. Some questionable Competitive local exchange carrier (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 dialled up party. Since most carriers charge their customers on the basis of caller ID, some VoIP providers use Caller ID replacement to save local plan minutes. In addition ISDN (PRI) based PBX's (in many cases) can send whatever number(s) they wish to be displayed at the called parties end. This may be their main switchboard number, or the dial-able (DID) extension number that placed the outgoing call.
Prefixing the following numbers will disable Caller ID on a call-by-call basis:
Other countries and networks vary, however on GSM mobile networks, callers may dial #31#[16] before the number they wish to call to disable it.
Depending on the operator and country, there are a number of prefix codes that can unblock Caller ID.
On GSM mobile networks, callers may dial *31#[16] before the number they wish to call to enable caller ID.