Caller identification (Caller ID) is a telephone service, available in analog and digital telephone systems, including voice over IP (VoIP), 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 Calling Name Presentation (CNAM). The service was first defined in 1993 in International Telecommunication Union—Telecommunication Standardization Sector (ITU-T) Recommendation Q.731.3.
The information received from the service is displayed on a telephone display screen, on a separately attached device, or on other displays, such as cable television sets when telephone and television service is provided by the same vendor.
Caller ID service is variously known by many similar terms, such as CID, calling line identification (CLI, CLID), calling number delivery (CND), calling number identification (CNID), calling line identification presentation (CLIP), and call display.
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 concept of calling number identification 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, caller ID and ANI are not equivalent services. ANI was originally a service in a non-electronic central office that identified the telephone number of the line from which a call was originated. Previous to this, the calling number could not be identified electronically. In addition to the caller's telephone number, caller ID may also transmit the subscriber's name, when available. The name can be passed on by the originating central office, or it is obtained from a line information database by the terminating switch. If no name is available, 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.
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), then 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 "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. Cellphone companies must support interconnecting trunks to a significant number of wireline and PSTN access carriers.
Calling line identity (CLI) localisation is the process of presenting a localised calling line identity to the recipient of a telephone call. CLI localisation is used by various organisations, including call centres, debt collectors and insurance companies. CLI localisation allows companies to increase their contact rate by increasing the chance that a called party will answer a phone call. Because a localised CLI is displayed on the called party's device, the call is perceived as local and recognisable to the caller rather than a withheld, unknown or premium rate number. The presented telephone number is adjusted depending on the area code of the dialed number.
In 1968, Theodore George "Ted" Paraskevakos, while working in as a communications engineer for SITA in Athens, Greece, 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 was transmitted to the receiver's device. This method was the basis for modern-day Caller ID technology.
From 1969 through 1975, Paraskevakos was issued twenty separate patents related to automatic telephone line identification, and since they significantly predated all other similar patents, they appear as prior art in later United States patents issued to Kazuo Hashimoto and Carolyn A. Doughty.
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. 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, such as the caller's name, to the receiving apparatus and to make banking by telephone feasible. 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 one thousand 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 Institution, National Museum of American History 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 London, Ontario, promoted the idea of displaying caller ID on a telephone. 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.
In 1977, the Brazilian inventor Valdir Bravo Salinas filed a patent application for a caller ID device at the Brazilian Patent and Trademarks Office (INPI). The patent was issued in 1982 as patent PI7704466 and is the first patent issued for a caller ID equipment in Brazil. Later in 1980, two other Brazilian inventors, João da Cunha Doya and Nélio José Nicolai, filed patent applications for other caller ID devices. Doya’s application was filed on May 2, 1980 and issued as patent PI8003077. Nicolai’s application was filed on July 2, 1980 and rejected for being a copy of Salinas' invention. In 1981 another application for a caller ID equipment was filed at the INPI by José Daniel Martin Catoira and Afonso Feijó da Costa Ribeiro Neto. This application was granted and the patent issued as patent PI8106464.
The first market trial for Caller ID and other "Custom Local Area Signaling Services" (CLASS) was conducted by Bell Atlantic in May 1984. Bell Communications Research (BellCore) named the service "Caller ID". The other regional Bell operating companies later adopted the name and eventually became the generally accepted name in the United States. 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 Bell Atlantic. 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, followed by US West Communications (now CenturyLink) 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 (not a new feature)
- 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.
data checksum digit d1 d2 d3 d4 s1 s2 s3 s4 1 1 0 0 0 1 1 1 1 2 0 1 0 0 0 1 1 1 3 1 1 0 0 1 0 1 1 4 0 0 1 0 0 0 1 1 5 1 0 1 0 1 1 0 1 6 0 1 1 0 0 1 0 1 7 1 1 1 0 1 0 0 1 8 0 0 0 1 0 0 0 1 9 1 0 0 1 1 1 1 0 0 0 1 0 1 0 1 1 0 * 1 1 0 1 1 0 1 0 # 0 0 1 1 0 0 1 0 A 1 0 1 1 1 1 0 0 B 0 1 1 1 0 1 0 0 C 1 1 1 1 1 0 0 0 D 0 0 0 0 0 0 0 0
FSK mark= 1200 Hz space= 2200 Hz 1200 bpsk
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 may not be transmitted to the recipient. Also, in the United States a caller may block the number they are calling from by dialing *67 before dialing the phone number. This will not work when dialing an "800" number where the receiver of the call pays for the call.
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 (such as in the UK) use 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 full phone number (including area code, and including international access code and country code if it's an international call). 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. A query with Signalling System 7 (SS7) query may be initiated by the called party's central office to retrieve the information for Calling Name delivery to the caller ID equipment at the subscriber'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. This CNAM dip fee is very small – less than a penny per call. AT&T starts their negotiations for CNAM dip fees at about $.004 per lookup. OpenCNAM fees are a bit more expensive, up to $.0048 per lookup. 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 toll-free numbers, they may report a string such as TOLLFREE NUMBER if the name is not available in a database.
Telemarketing organisations often spoof caller ID. In some instances, this is done to provide a "central number" for consumers to call back, such as a toll-free 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 misleading caller ID. Individuals may file civil suits and the Federal Communications Commission (FCC) can fine companies or individuals for illegally spoofing or blocking caller ID.
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 ID transmission is implemented using different technologies and standards in some countries. In the United States the Bellcore FSK standard is prevalent, whereas Taiwan uses ETSI FSK. Sometimes individual service providers within a country use different standards. Caller ID converters can be used to translate from one standard to another.
|Country||Caller ID standard|
|Brazil||Bellcore FSK / V23 FSK / DTMF|
|China||Bellcore FSK / DTMF|
|Hong Kong||Bellcore FSK|
|Ireland||ETSI FSK V23 (ETS 300 659-1) Ring Pulse Alert Signalling. Data sent after first short ring.|
|Japan||V23 FSK / DTMF|
|New Zealand||Bellcore FSK|
|Taiwan||DTMF / ETSI FSK|
|United Kingdom||SIN227 (V23 FSK before first ring)|
|United States||Bellcore FSK|
Telephone equipment usually displays CLID information with no difficulty. Modems are notoriously problematic; 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. Other UK telephone companies use slight variations on the Bellcore standard, and CLID support is "hit and miss".
CND is currently available in Australia to subscribers to the Integrated Services Digital Network (ISDN). There is a legislation under section 276 of the Australia Industry Code - Calling Number Display (ACIF C522: February 2003).
In the United States, telemarketers are required to transmit caller ID. This requirement went into effect on January 29, 2004. It is generally illegal to spoof Caller ID if done "with the intent to defraud, cause harm, or wrongfully obtain anything of value". The acts are prohibited under Truth in Caller ID Act of 2009.
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 in the United States in 2007[update] made caller ID spoofing illegal for fraudulent purposes.
In March 2017 the FCC approved a new rule that would allow telecom companies to block robocallers that use fake caller ID numbers to conceal their true location and identity. The rule means telecos can block robocallers at the network level, long before a call passes through a carriers network and arrives at a subscriber's house or business. T-Mobile was the first major US carrier to announce plans to implement blocking technologies based on the new rule.
Starting in mid-2017, and with intended culmination in 2019, the FCC pushed forward Caller ID certification implemented via a methodology of SHAKEN/STIR. This initiative was further strengthened by the TRACED Act, enacted in December 2019.
Blocking and unblocking caller ID
Caller ID blocking is the common term for a service by which a caller can prevent the display of the calling number on the recipient's telephone.
Blocking the number is formally referred to as calling line identification restriction (CLIR).
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. The telephone service provider may also have vertical service codes which can be dialed to configure blocking as active for all calls or on a call-by-call basis.
In some locations in the United States, regulations allow (or require) blocking to be automatic and transparent to the caller.
Where blocking is applied on a call-by-call basis (that is, at the time a call is made), subscribers can block their caller ID by dialing a special code (a vertical service code, or VSC) before making a call. In North America and some other regions, the code is *67 (1167 rotary dial), while in the United Kingdom and Ireland, it is 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 PRI.
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 name, number (or both) is blocked. Some telephone companies protect their clients from receiving calls with blocked information by routing anonymous calls to a service (such as AT&T Privacy Manager), where the caller is required to announce himself or herself. The service then asks the called party if they want to accept or reject the call. Other telephone companies play a recording to the caller advising them of the called party's rejection configuration, and often offer advice (such as prefixing their dialing with *82) on how to get their call to the intended called party. 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, whereby subscribers must purchase additional services in order to cancel out other services.
Disabling caller ID delivery
Depending on the operator and country, there are a number of prefix codes that can block or disable Caller ID transmission by the caller. Prefixing a telephone number with the following codes disables Caller ID on a per-call basis:
|Albania||#31# (cell phones)|
|Argentina||*31# (landlines) or #31# (most cell phone companies)|
|Australia||#31# (mobile phones) 1831 (analogue landline) *67 (NBN landline)|
|Brazil||#31# (mobile phones)|
|Bulgaria||#31# (mobile phones)|
|Canada||#31# (mobile phones) or *67 (landlines)|
|France||#31# (cell phones) or 3651 (landlines)|
|Germany||On most landlines and mobiles, *31#, however some mobile providers use #31#.|
|Greece||*31* (landlines), #31# (cell phones).|
|India||#31# after network unlocked|
|Ireland||#31# (dialling from mobile) 141 (dialling from landlines)|
|Israel||*43 (landlines) or #31# (most cell phone companies)|
|Italy||*67# (landlines) or #31# (most cell phone companies)|
|Netherlands||*31*, #31# (KPN)|
|New Zealand||0197 (Telecom/Spark), *67 (Vodafone), #31# (2degrees)|
|North America||*67, 1167 (rotary phone), #31# (AT&T Wireless)|
|South Africa||*31* (Telkom)|
|South Africa||#31# (Cell Phones)|
|South Korea||*23 or *23# (most cell phone companies)|
|Spain||#31# (Cell Phones); 067 (landlines)|
|Switzerland||*31# (or *31+Targetnumber -> Call-by-Call disable) (landline)|
#31# (or #31+Targetnumber -> Call-by-Call disable) (mobile)
Some countries and network providers do not allow Caller ID blocking based on the domestic telecommunications regulations, or CLIR is only available as an external app or value-added service.
Enabling caller ID delivery
Depending on the operator and country, there are a number of prefix codes that can unblock or enable Caller ID transmission by the caller.
|Australia||*31# (mobile phones) 1832 (analogue landline) *65 (NBN landline)|
|Czech Republic||*31* (landline)|
|Germany||*31# (Some mobile providers)|
|Ireland||*31# (dialling from mobile)|
142 (dialling from landlines)
|New Zealand||0196 (Telecom/Spark)|
|North America||*82 (*UB, UnBlock)|
1182 (rotary phone).
On GSM mobile networks, callers may dial *31# to enable caller ID on all subsequent calls.
Caller ID spoofing
Caller ID spoofing is the practice of causing the telephone network to display a number on the recipient's caller ID display that is different than that of the actual originating station. Many telephone services, such as ISDN PRI based PBX installations, and voice over IP services, permit the caller to configure customized caller ID information. In corporate settings this permits the announcement of switchboard number or customer service numbers. Caller ID spoofing may be illegal in some countries or in certain situations.
Dip fee fraud
A consumer's telephone company must pay a small fee for the Caller ID text that is transmitted during a call. The fee is called a CNAM dip fee. It is named a dip fee because the called party's carrier pays a fee to dip into the originating telephone company's database to get the Caller ID information.
Several companies engage in generating dip fees by catering to companies that make a large number of outbound calls. CallerId4U and Pacific Telecom Communications Group cater to telemarketers and generate revenue on fees from Caller ID information. The telemarketers enter into an agreement with companies like CallerId4U and Pacific Telecom Communications Group and share the revenue produced during the telemarketing call.
Dip fees vary wildly. According to Doug McIntyre, the wholesale rates are on the order of $0.002 to $0.006 per database dip. And according to Aaron Woolfson, president of TelSwitch Inc, the fee structure for dip fee fraud can include:
- the carriers pay a fee of $0.003 per call or $300 per 100,000 calls to the database owner
- the database owner pays the number dealers $0.0024 per call or $240 per 100,000 calls
- the number dealers share revenue with the robocaller $0.00096 or $96 per 100,000 calls
Consumers face significant barriers to exiting a call list and often cannot have themselves removed from the list. Calling the opt-out numbers often results in a fast-busy so the call never completes and the consumer remains on the list.
According to reports companies like CallerId4U has thousands of phone numbers and thousands of FTC complaints filed against them each month for violating Do Not Call registration. The large number of phone numbers dilute the number of complaints against the company and phone number.
- The inverse feature, giving the number originally dialed, is known as direct inward dialing, direct dialing inward, or Dialed Number Identification Service. 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.
- Tu, Huahong; Doupé, Adam; Zhao, Ziming; Ahn, Gail-Joon (September 2017). "Toward Standardization of Authenticated Caller ID Transmission" (PDF). IEEE. Retrieved February 27, 2019.
- "CLI localisation Under the Microscope". Nexbridge. March 30, 2016. Archived from the original on March 30, 2016. Retrieved February 24, 2019.
- Formerly known as Société internationale de télécommunication aéronautique
- Patent #3,727,003/4-10-1973 and Patent # 3,812,296/5-21-1974
- Patent # 4,242,539/12-30-1980
- Patent # 4,551,581/11-5-1985 and Patent # 4,582,956/4-15-1986; (both assigned to AT&T Bell Laboratories)
- PhoneTel Patent Services :: History : Hashimoto Archived 2007-07-01 at the Wayback Machine
- Hashimoto, Kazuo; Kilby, Jack. "PhoneTel Collection" – via siris-archives.si.edu Library Catalog.
- "Fight heats up over patents on Caller ID. (Kazuo Hashimoto)". 1 April 1995. Archived from the original on 20 November 2008. Cite journal requires
- "Caller ID - Consumer's Friend or Foe? - NYTimes.com". nytimes.com. 4 April 1992. Retrieved 3 February 2015.
- "47 CFR 64.1601" (PDF).
- "Caller ID Spoofing". Federal Communication Commission. 26 September 2018. Retrieved 28 January 2019.
- "Caller ID FAQ". Marilyn Ainslie. 1 April 2004. Archived from the original on 14 February 2015. Retrieved 3 February 2015.
- "Telecom New Zealand TNA 102" (PDF).
- "How to modify your modem driver file". Talking Caller ID. Archived from the original on 7 February 2019. Retrieved 11 May 2020.
- "Caller ID FAQ". Marilyn Ainslie. 1 April 2004. Archived from the original on 21 March 2016. Retrieved 21 March 2016.
- "Caller Identification". AMTA. Archived from the original on 15 August 2018. Retrieved 11 May 2020.
- 18 FCC Rcd 14014 (FCC, July 3, 2003) at para. 173 et seq.
- 47 C.F.R. § 64.1601(e).
- State v. Schuette, 273 Kan. 59, 44 P.3d 459 (Kansas 2002)
- Sawers, Paul (March 24, 2017). "T-Mobile kicks off industry robocall war with network-level blocking and ID tools". VentureBeat. Retrieved November 23, 2019.
- Fiegerman, Seth (August 19, 2016). "Apple, Google, Microsoft join 'strike force' to fight robocalls". CNN Business. Retrieved November 23, 2019.
- Pai, Ajit (2017). "Combating Spoofed Robocalls with Caller ID Authentication". FCC.
- Brodkin, Jon (Feb 14, 2019). "Ajit Pai orders phone companies to adopt new anti-robocall tech in 2019". Arstechnica.
- Trump signs the TRACED Act, the first federal anti-robocall law
- Price, Leigh (19 June 2012). "HOW TO: block your number when calling someone". Telstra Corporate Affairs. Retrieved February 19, 2019.
- "PCS Sites Redirect". www.arcx.com. Archived from the original on 2008-02-03. Retrieved 2008-02-12.
- Unuth, Nadeem (December 20, 2018). "The Best Call Blocker Apps For Smartphones". Lifewire. Retrieved February 18, 2019.
- Panagia, Adam. "Caller ID Spoofing 101 and What To Do About It". AT&T Cyber Aware News and Information. Retrieved February 28, 2019.
- "Over 100,000 FTC Complaints Filed Against CallerID4U, Inc". The Telecom Compliance News Press. January 22, 2013. Retrieved February 27, 2019.
- JD (January 22, 2013). "CallerId4U, Inc. - Millions of Illegal Telemarketing Calls". 800 Notes. Retrieved February 27, 2019.
- Krouse, Sarah (June 4, 2018). "Why Robocallers Win Even if You Don't Answer". The Wall Street Journal. Retrieved November 23, 2019.
- MacIntyre, Doug (February 3, 2014). "Caller ID information wrong". Newsgroup: comp.dcom.telecom. Usenet: WIKdnRBVbcvWzm3PnZ2dnUVZ_oSdnZ2d@giganews.com. Retrieved November 26, 2019.