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Caller ID

Caller ID

Caller identification (caller ID) is a telephone service for analog and digital telephone systems, including voice over IP (VoIP), that transmits a caller's telephone number to the called party's telephone when the call is being set up. The service may include the transmission of a name associated with the calling telephone.


Caller ID, short for caller identification, is a telephone service that transmits a caller's telephone number and the transmission of a name associated with the phone number to the called person's telephone. The service is available for both analog and digital phone systems, including voice over IP (VoIP). Caller ID was widely introduced in the 1990s and has since become ubiquitous in all telephone services—especially with the introduction of smartphones and other telephone services, such as SMS and MMS, which rely on caller ID for identification, along with other applications and services.

Caller ID is not regulated by a governing body, such as the Federal Communications Commission. Rather, the services run on a patched-together network of decentralized databases that often operate with outdated information. Caller ID works to provide the name and number of a calling party where available, but the definition of caller ID only needs to supply the phone number of the calling party. The usage of its term has made it synonymous with the calling name as well, although this is technically a separate service.

Benefits of caller ID

The benefits of caller ID include allowing consumers to know who is calling before they answer the phone, giving them the ability to screen calls and answer important calls while ignoring unimportant or possibly fraudulent calls. Further, caller ID offers consumers a record of recent calls for access to the phone numbers of those who have called, and gives businesses a chance for a callback in appropriate cases. This can be especially helpful in the case where a call has been missed, but the caller has not left a voice message. Furthermore, in a larger context, caller ID allows calls to be tracked and allows the appropriate person to answer a call.

For corporations, caller ID also offers an opportunity to develop a caller database and learn more about customers through caller data. As well, the data can be used to personalize the customer experience and increase the appeal to different customers. This can include using a caller ID solution to ensure a business makes an impression on the consumer it is calling right from the caller ID display, before the call has been answered.

Differences by region

Caller ID standards for data transmission and signaling have been published by various standards committees, regulatory agencies, and network carriers. They tend to fall into one of two categories, based on how the caller ID data is transmitted: FSK modulation or DTMF. While the United Kingdom uses BT and CCA recommendations, British Telecom developed a different recommendation that uses FSK modulations for data transmission. Whereas DTMF-based caller ID is used in countries including Brazil, Belgium, Denmark, Holland, Finland, Iceland, India, the Netherlands, Saudi Arabia, Sweden, and Uruguay.

Other countries will have deviations from these main categories, which are available in country-specific PSTN standards. The Telcordia recommendation is used in countries such as the United States, Canada, Australia, China, Hong Kong, New Zealand, and Singapore, with different data formats and deviations in signal characteristics. These variations are based on the Bellcore FSK standard. And most deviations are based on network transmission interfaces, including power levels and impedances or timing variations by country.

As a result of these regional differences, phones purchased in one country may not be compatible with local caller ID standards when used in a different country. For example, the standard used in the United States is the Bellcore FSK, while Taiwan uses ETSI FSK; so a phone purchased in one country will not understand the other country's caller ID standard. However, caller ID converters for translating between one standard and another can be utilized to provide a consistent experience. Caller ID standards in specific countries include the following:

Regional differences in caller ID standards

Caller ID standard


Bellcore FSK/V23 FSK/DTMF


Bellcore FSK


Bellcore FSK/DTMF

Hong Kong

Bellcore FSK


ETSI FSK V23 (ETS 300 659-1)

Caller ID technologies
Calling line identification

Caller ID consists of two user-facing identifiable pieces of information. The first is a phone number, which is also referred to as calling line identification of CLID. This is the public switched telephone number (PSTN) presented as the identification of the caller. The second part is a caller party name, also referred to as CNAM. The information is displayed on a telephone's display, or a separately attached device, through a modem that passes CLID information to a computer for the purposes of call logging and blocking. However, the systems of modems can be different between countries, which can cause hardware and software incompatibilities in international calls. Although most modems are designed and programmed to handle multiple signaling methods for different standards.

When CLID, and by extension caller ID, was developed, the world of telephony was primarily a realm of Plain Old Telephone Service (POTS) lines, and the caller information was tied to a Central Office switch that connected the wires. This period had fewer carriers at the time, and caller identification was easy to maintain and store. And unlike phone numbers or domain name services (DNS), both of which are internationally recognized databases and act as authoritative sources, there is no central authority or regulation for caller ID, which in part creates a vulnerability and capability for caller ID spoofing.

For landlines, the displayed number on caller ID corresponds to the phone number registered with the line. For a PRI or SIP connection, the phone system can control what number is displayed. The caller ID name can also be determined by the receiving carrier. This is done by the receiving carrier querying a CNAM database to determine the name registered to the provided phone number. This allows for the information on calls from a voice over IP (VoIP) system to be modified, with a web interface allowing the caller to edit this information. When routing calls, this feature can make it easier for customers to reference callback numbers and identify potentially important calls.

Calling Name (CNAM)

Calling name, also stylized as Calling NAMe and shortened to CNAM, is a third-party service used by carrier to identify and display the name of inbound callers. There is no universal standard for the collection and accuracy of the information in these databases, with multiple possible databases available to carriers with either redundant or conflicting information based on how maintained a database is. This can create inconsistencies with how names are displayed, with the displayed information varying depending on who receives the call and their phone provider.

For those companies maintaining CNAMs, they maintain a private database on the phone number and name pairs in the United States and abroad. A caller ID outputs up to 15 ASCII characters to display a name, and a typical display name is 9 to 12 characters, while the phone number is usually displayed as a ten-digit number.

Dialed number identification service (DNIS)

Dialed number identification service (DNIS) is a service offered by telecommunication network providers to help the call receiver to identify the number the caller dialed. This is done by transmitting the dual tone, multi-frequency digits the caller dialed to a destination where a mechanism decodes the signals and either displays them or makes them available for use by devices at the receiving end.

This technology is useful for contact centers, where a PBX receives calls dialed to different 800 or 900 numbers on the same port. The DNIS data would contained the dialed number, and enable the PBX to track the call. Integrating call processing and call routing with the associated data then helps with the possibility for first call resolution and an improved customer experience and satisfaction.

Automatic number identification (ANI)

Automatic number identification (ANI) is a service that allows the receiver of a phone call to capture and display the phone number of the originating call and is mainly used for billing purposes. Previous to this service, telephone operators would have to request the number of a calling party for a toll call. ANI is somewhat similar to caller ID in function, but the underlying technology is different. Although the method of transmitting phone number data differs based on the service provider, it is often sent as a digital tone with the phone call. ANI is also more resistant to blocking than traditional caller ID systems.

ANI is another call center technology that can be used to help impact the customer experience. For example, with ANI services, contact centers are able to route calls based on location, with the area code of the call used to identify its probable location. Or, in a separate scenario, a phone number can be used to query a customer database and the call can be routed according to customer characteristics. ANI can also be used, with a customer database, to provide agents with customer account information with a call, rather than requiring the agent to manually manage the information. This can allow an agent to acknowledge the customer and personalize the call.

Network caller ID (NCID)

Network caller ID (NCID) is an open-source client/server network caller ID package that consists of a server called NCIDD, or NCID daemon; a universal client called NCID; and multiple client output modules and gateways. The server, NCID, monitors either a modem, device, or gateway for the caller ID data, which is collected and sent, via TCP, to any connected devices. Many devices, including smartphones, and services can detect caller ID information. An NCID gateway collects the caller ID data from these sources and passes it on to the main NCID server. From there, the caller ID data is distributed to all connected clients, similar to caller ID information collected from a traditional modem.

One example of a non-modem device is voiceover IP (VoIP) service that collects caller ID data as SIP packets. Another example is the Whozz Calling series of ethernet link devices that obtain caller ID information from multiple plain old telephone service (POTS) lines.


The NCID protocol is a simple, human-readable ASCII text consisting of field pairs, which includes a field label and the labels associated field data, using an asterisk character as a delimiter. Transmission between the NCID server and its clients is usually done through TCP or IP, and usually over port 3333. Additional field pairs have been added as the NCID servers have been enhanced with new features and support for more devices. NCID servers can also be enhanced with hardware that can supply caller ID data through a NCID server accessing device directly through the RS232 serial port or through USB, or can be accessed indirectly through NCID gateways, which are usually scripts and programs included with an NCID package.


Traditionally, AT-compatible modems expect telecommunications carriers to send caller ID data as either single data message format (SDMF) or multiple data message format (MDMF). The modem then decodes the data stream into human readable text, which the NCID server can then parse. And if supported by a modem, an NCID server can also decode the raw SDMF or MDMF data stream.

Companies offering Caller ID services

Caller ID for call centers and customer relationship management

Caller ID integrated into customer relationship management (CRM) software has increased in use as it offers a chance for companies and call centers to develop a more efficient service pathway with greater personalization. Using caller ID for inbound calls offers a chance to identify a customer's identity before an agent answers the call. This can, depending on the integration, display the caller's name, email address, where they work, and even a complete interaction history.

For outbound calls, the use of software can change and present a specific caller ID for outbound calls. In the case of a third-party call center, this can be to present the appropriate business name and callback number. This can also present a chance for a business to even provide a reason for calling in some cases. The use of outbound caller ID can increase call pickup rates and improve brand recognition in some cases.

As well, with an increased use of SMS in CRM software, and with smartphones offering a chance to display a caller ID when sending an SMS message to a consumer, has come an increase in consumer trust when receiving an SMS message from a business, and an increase in interaction rates.

Caller ID Spoofing

While there are legitimate purposes for altering caller ID information when placing a call—such as a call center placing calls on behalf of multiple clients who wish to have the accurate information displayed for a client, or a doctor using a general call back number for display when calling a patient for directing future inquiries—there has also been an increase in caller ID spoofing for scam call and fraudulent call purposes. This has become one of the largest complaints to communications commissions, such as Canada's Canadian Radio-Television and Telecommunications Commission (CRTC) and the United States' Federal Communications Commission (FCC).

Caller ID spoofing is the act of a specific person or group changing the information that appears on the caller ID display to misrepresent themselves and trick those answering the call. Spoofing can be done in the following ways:

  • The caller ID may be altered to match the first 6-digits of a telephone number to look like a local call, even a call from a neighbor, a practice also known as neighboring
  • The caller ID may display a user's own phone number, a practice also known as mirroring
  • The caller ID may display the number of another individual or another organization to pose as a recognizable brand or person
  • The caller ID may be altered to represent a number that cannot be dialed with the telephone network

The changing technologies of telephone networking, especially the development of VoIP services, have also offered new technologies for caller ID spoofing and scammers using these technologies to trick consumers. Previously, telecommunication networks had tighter regulations and fewer problems with scammers and the large telecommunication companies dominated these networks and helped reduce the possibility of caller ID spoofing. VoIP services also saw many telecommunication companies enter the marketplace without the infrastructure necessary to protect consumers from robocalls or else they ignored the problem. In 2018 alone, it was reported that Americans received 26.3 billion robocalls.

The increase in robocalls and scam calls using caller ID spoofing has led to governments working with telecommunication companies to implement call authentication technologies to combat the caller ID spoofing. One of these call authentication technologies is the STIR/SHAKEN protocol, which works to provide a technological framework for carriers to authenticate calls at the source and warn consumers about potentially fraudulent callers before a consumer picks up the call.

Telecommunication companies have also begun using data metrics in order to pinpoint locations and bad actors likely to spoof caller ID in order to steal personal and financial information from consumers. As part of this, the TRACED act, which mandated that the STIR/SHAKEN protocol needed to be adopted by all carriers by June 2021 in the United States, also mandated the customers should receive robocall blocking at no additional charge. The act also gave the FCC an opportunity to push telecommunication companies for other technical solutions to block unwanted calls for consumers.


The Secure Telephone Identity Revisited and Signature-based Handling of Asserted Information Using toKENs (STIR/SHAKEN) protocol proposed and included in the TRACED act is inspired by the HTTPS web communication protocol and is an attempt to apply the same approach from web browsers to telephones. However, the proposal has been considered by some to have two major drawbacks. The first is that the STIR/SHAKEN protocol requires a public key infrastructure (PKI), which can be an expensive infrastructure to set up and to maintain. And regardless of the cost and possible operational issues associated with a PKI, which acts as a globally trusted certificate authority, is also unclear. The second is that the STIR/SHAKEN protocol is designed to work with the SIP system for VoIP, leaving SS7 systems (landline and mobile phones) out of scope, which has been perceived to limit the overall effectiveness of the proposed protocol.

The STIR/SHAKEN protocol works by using a cryptographic certificate to sign a caller's ID. Calls are signed by the telecommunication network where the call originated, and are intended to be verified by the voice provider that connects the call, through a third-party remotely-hosted certificate repository. Work on the protocol began in the mid-2010s, with an adoption rate increasing in 2018 when the FCC imposed a hard deadline for the adoption of the STIR/SHAKEN protocol and more robust authentication inside telecommunications.

United States telecommunications companies began testing the protocols in their networks, with AT&T and Comcast announcing the first successful STIR/SHAKEN-authenticated call between two different networks in March 2019.

An alternative method has been created to achieve end-to-end authentication of caller IDs for both SIP and SS7 systems without requiring any PKI. One such idea has been to leverage the DTMF signaling in a call-back session as a trusted channel to send code to the caller, in conjunction with a password authenticated key exchange (PAKE) protocol to perform a key exchange over the data channel, in order to establish a shared high-entropy session key used to authenticate the caller ID end-to-end.

Comcast and AT&T

Comcast and AT&T have both used the STIR/SHAKEN protocol in existing caller ID and authentication services. Comcast adapted the STIR/SHAKEN protocol to develop a Verified Caller ID feature for the company's Xfinity Voice, Comcast Business Voice, and Business VoiceEdge Select customers. The service works to protect those customers from robocalls and caller ID spoofing, with the service working to validate all calls coming to those customers and display a Verified label when the service deems the call to be authentic and not spoofed.

Similarly, AT&T, which previously offered a Call Protect service that worked to use data analytics on calls and consumer reports in order to evaluate the authenticity of calls, integrated the STIR/SHAKEN protocol into the Call Protect service. The data derived from the protocol is intended to act as another data point in the Call Protect service, to help automatically block calls that the service categorizes as "fraud" and offer warnings for suspected "spam" calls, which AT&T will block in a premium version of the Call Protect Service.


In a different direction, Verizon has developed the company's Neighborhood Filter to the company's Call Filter application, which works to block neighborhood spoofing, which is a tactic in which spam callers use a local or neighborhood number to get people to pick up calls. Verizon's solution is to prevent all calls, even legitimate calls, from area codes within a chosen neighborhood, unless the number has been previously saved in a phone. This a further feature of the company's Call Filter service, which works to detect and block calls when Verizon detects a possible spam call, and allows users to report unsolicited numbers and block robocalls based on preferred level of risks. This application is also run on the STIR/SHAKEN protocol to further increase the ability to detect unverified calls.

T-Mobile and Rich Call Data

T-Mobile has developed similar STIR/SHAKEN integrations into the company's proprietary offerings. One such integration has included the use of Rich Call Data (RCD). This technology is used to show details such as branding of a company calling for the person called, and this information is expected to also allow those organizations include a "why" they are calling. This has been used as well in combination with the STIR/SHAKEN protocol to provide a further level of call authentication.

This verification is done using the STIR/SHAKEN framework, which helps providers digitally verify the authenticity of numbers originating and travelling across networks and to verify that the call is coming from the number displayed on the caller ID before the call reaches a consumer. RCD has been used to allow an organization to provide data for an originating call and relies on industry-backed centralized registry, rather than third-party database on the receiving end of the call. This is to further ensure information will only be displayed with verification of the authenticity of the call by the network.

T-Mobile's RCD integration with STIR/SHAKEN was tested for a proof-of-concept with participation with First Orion, CTIA, Everbridge, iconectiv, NetNumber, Numeracle, and Twilio. The proof-of-concept was pursued to show the telecommunications and caller ID third-party companies can work together for increased consumer protection and confidence. This is considered an important feature for T-Mobile, which has pointed to data from Pew Research Center, which found 80 percent of adults in the United States do not answer calls if the number is not a known number.

These developments also come as T-Mobile's services, including Scam Shield and Metro by T-Mobile, work to protect consumers from scam calls with automatic scam call warnings. As well, research has shown that T-Mobile's network has been shown to be 30 percent better at detecting scam calls than other major networks.

Further call authentication strategies

While caller ID displays information about the person calling, it was never intended to stop those calls from occurring. Developers have since created call blocking technology to identify and stop calls from happening once specific criteria is met. These call blocking protocols exist between a third party as calls are received. Phone companies and applications on mobile devices are able to provide these service as call blocking applications or call screening features.

These services work to recognize the source of an incoming call and allow or deny it based on predefined preferences. And with the increase in smartphone technology, software developers are able to make consistent and incremental improvements and advancements in call blocking, with many applications available that work seamlessly to screen and block phone calls for the user.

There is a difference in many cases between blocking calls, which many of these services offer, and flagging them. Call blocking is normally for one-off scenarios for any reasons, while spam flagging seeks to report fraudulent and non-compliant businesses to national databases. This means as more people flag a phone number, eventually it finds its way into call blocking services automatically, which in turn lessens the workload of individuals creating block lists and can help promote better business practices, as it puts an impetus on a business to monitor the integrity of their phone number operation.

History of Caller ID

Despite caller ID's more widespread consumer adoption not happening until the 1990s, where the inclusion of small screens in telephones for the purpose of screening calls was more widely adopted, the technology was originally developed during the late 1960s and early 1970s. During this period, Theodore Paraskevakos, a Greek immigrant to the United States, patented a new technology that found a way to transfer data via the telephone lines through electronic signals. This technology gave way to transferring specific signals, or words, across telephone lines.

By the mid-1970s, Japanese inventor Kazuo Hashimoto built the first variant of an actual caller ID device. This led to the adoption of caller ID in business and call center applications, before the technology became a popular fixture of home phones. The adoption of cell phones and eventually smartphones, in the late 1990s and early 2000s, came with an increase in the importance of caller ID, as it was being made a part of receiving calls and SMS or MMS messages. And with smartphones, it became increasingly odd not to know who was calling.

Evolving telecommunications

The telephone and telecommunications infrastructure has evolved to include three primary methods for public telecommunications:

  • Public switched telephone network (PSTN)
  • Integrated services digital network (ISDN)
  • IP Telephone/voice over IP (VoIP)

PSTN is the combination of various telephone networks used by national, regional, and local telephone operators. These networks provide the infrastructure and services for public telecommunications, including telephone lines, cellular networks, satellites, and fiber optic cables. This is considered the infrastructure for making calls. PSTN used to be a network of analog systems and has been switched over time to an almost entirely digital network. This switch from analog to digital infrastructure has created vulnerabilities that scam or spoof callers have used.

The PSTN works through a network of "switches" that route calls from one destination to the next. Once the call is engaged on a landline or traditional cellular service, the call is handed off through several destinations, which include fiber optic cables, cellular towers, and satellites. These networks then transfer the communications from one location to the next. The United Kingdom has already committed to retiring the country's PSTN in 2025 and replace it with a digital "all-IP" network.


ISDN is similar to PSTN, but with the main difference being that the ISDN network is capable of transferring data as well as speech through multiple location points. This technology was originally developed in the late 1980s, when the telephone system primarily transferred voice or speech, previous to the development of ISDN. The technology is capable of transferring data in packets from location to location through the PSTN network. ISDN involves several types of interfaces:

  • Basic rate interface (BRI)
  • Broadband ISDN (B-ISDN)
  • Narrowband ISDN (N-ISDN)
  • Primary rate interface (PRI)

One of the benefits of ISDN, compared to PSTN, is the ability to transfer data and make calls clearer, and the network technology has since become a preferred communications method for many telecommunications companies.


The newest of the telecommunication technologies, VoIP uses broadband networks to transport voice communications from the caller to the recipient. This works through the conversion of audio packets of data via codecs that are transmitted over an IP network. VoIP does not rely on the circuit-switched network that phones traditionally use and is considered less expensive for telecommunications companies as a result. VoIP is also the most popular communication method for many internet users.


October 4, 2021
Spoofing scams are Metro Bank's October scams of the month.
September 29, 2021
Phone companies must now block carriers that didn't meet FCC robocall deadline.
September 29, 2021
Cybercriminals bypass 2FA and OTP with robocalling and Telegram bots
September 27, 2021
Humboldt County Sheriff's Office warns of common phone scam targeting local residents.
September 25, 2021
Report: Only one-third of largest phone companies have completely adopted anti-robocall technology.
September 22, 2021
VL Telecom announces a partnership with Truecaller Verified Business Caller ID solution.
September 15, 2021
Truecaller is looking to raise $116 million in an initial public offering on Nasdaq Stockholm.
September 7, 2021
Caller ID spoofing scam spreads through hospitals in the Houston area.
August 6, 2021
Total robocalls decrease by 3% in the first month of STIR/SHAKEN release.
July 28, 2021
Google announces it's adding new caller ID, missed call, and SMS message features to Google Voice.

Further Resources


Call Authentication is Necessary for the Modern Telephony Network


August 26, 2020

Caller ID Spoofing | CRTC


April 7, 2021

FCC tells US telcos to implement caller ID authentication by June 30, 2021

Catalin Cimpanu


March 31, 2020

Frequently Asked Questions About Caller ID



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December 31, 2019
The measure should help reduce the torrent of unwanted calls, though it won't make them all disappear. Here's what you need to know


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