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Direct inward dialing (DID), also called direct dial-in (DDI) in Europe and Oceania, is a
telecommunication Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...
service offered by telephone companies to subscribers who operate a private branch exchange (PBX) system. The feature provides service for multiple
telephone number A telephone number is a sequence of digits assigned to a landline telephone subscriber station connected to a telephone line or to a wireless electronic telephony device, such as a radio telephone or a mobile telephone, or to other devices f ...
s over one or more analog or digital physical circuits to the PBX, and transmits the dialed telephone number to the PBX so that a PBX extension is directly accessible for an outside caller, possibly by-passing an auto-attendant.


Plain old telephone service

For direct inward dialing service, the
telephone company A telephone company, also known as a telco, telephone service provider, or telecommunications operator, is a kind of communications service provider (CSP), more precisely a telecommunications service provider (TSP), that provides telecommunicat ...
provides one or more trunk lines to the customer for connection to the customer's PBX, and allocates a range of
telephone number A telephone number is a sequence of digits assigned to a landline telephone subscriber station connected to a telephone line or to a wireless electronic telephony device, such as a radio telephone or a mobile telephone, or to other devices f ...
s to the customer. Calls to such numbers are forwarded to the customer's PBX via the trunks. As calls are presented to the PBX, the dialed telephone number is signaled to the PBX with
Dialed Number Identification Service Dialed Number Identification Service (DNIS) is a service offered by telecommunications companies to corporate clients which identifies the originally dialed telephone number of an inbound call. The client may use this information for call routing to ...
(DNIS) using a prearranged, usually partial format, e.g., the last four digits. The PBX may use this information to route the call directly to a telephone extension within the organization without the need for an operator or attendant. The service provides inbound telephone service for many telephone numbers requiring only a limited number of physical telecommunication circuits to satisfy the average concurrent usage by the customer. Traditionally, DID service used analog circuits. In these types of DID trunks the
customer premises equipment In telecommunications, a customer-premises equipment or customer-provided equipment (CPE) is any terminal and associated equipment located at a subscriber's premises and connected with a carrier's telecommunication circuit at the demarcation poi ...
provided signaling battery. The central office equipment detects the level of the line and disables service if the circuit is not operational. This is the reverse arrangement from standard
plain old telephone service Plain old telephone service (POTS), or plain ordinary telephone system, is a retronym for voice-grade telephone service employing analog signal transmission over copper loops. POTS was the standard service offering from telephone companies from 1 ...
(POTS) lines for which the central office provides signaling and talk battery. Nowadays, it is far more common to deliver DID service on a Primary Rate Interface (PRI) circuit. The trunks for DID service are unidirectional, inbound to the customer PBX. However, the service may be combined with ''direct outward dialing'' (DOD) allowing PBX extensions direct outbound calling capability with identification of their DID telephone number. In the United States the feature was developed by
AT&T AT&T Inc. is an American multinational telecommunications holding company headquartered at Whitacre Tower in Downtown Dallas, Texas. It is the world's largest telecommunications company by revenue and the third largest provider of mobile tel ...
in the 1960s, patterned upon the earlier IKZ service of the
Deutsche Bundespost The Deutsche Bundespost (German federal post office) was a German state-run postal service and telecommunications business founded in 1947. It was initially the second largest federal employer during its time. After staff reductions in the 198 ...
in Germany.


Use in fax services

DID service is also used by
fax server A fax server is a system installed in a local area network (LAN) server that allows computer users whose computers are attached to the LAN to send and receive fax messages. Alternatively the term ''fax server'' is sometimes used to describe a pr ...
s. A telephone line is terminated at a telephone interface (fax modem) of a computer that runs fax server software. A set of digits of the assigned phone numbers are used to identify the recipient of the fax. This allows many recipients to have an individual fax number, even though there is only one fax machine available. Some
voice over IP Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for the delivery of speech, voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. The terms In ...
(VoIP) vendors have used one central, remotely located fax server as a means of offering
Internet fax Internet fax, e-fax, or online fax is the use of the internet and internet protocols to send a fax (facsimile), rather than using a standard telephone connection and a fax machine. A distinguishing feature of Internet fax, compared to other Intern ...
service to their clients. In theory, standards such as
T.38 T.38 is an ITU recommendation for allowing transmission of fax over IP networks (FoIP) in real time. History The T.38 fax relay standard was devised in 1998 as a way to permit faxes to be transported across IP networks between existing Group ...
should have allowed VoIP subscribers to keep their existing fax equipment working locally; in practice, T.38 at the subscriber's site offers no benefit if the upstream provider is
least-cost routing In voice telecommunications, least-cost routing (LCR) is the process of selecting the path of outbound communications traffic based on cost. Within a telecoms carrier, an LCR team might periodically (monthly, weekly or even daily) choose between ro ...
to gateways that do not support T.38 and cannot reliably send or receive fax/modem traffic. A fax server at a central location, connected directly to
public switched telephone network The public switched telephone network (PSTN) provides Communications infrastructure, infrastructure and services for public Telecommunications, telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that ...
(PSTN) T-carrier primary rate interface lines and using direct inward dial to identify the intended addressee can convert an incoming fax to an electronic document (such as
.tif Tag Image File Format, abbreviated TIFF or TIF, is an image file format for storing raster graphics images, popular among graphic artists, the publishing industry, and photographers. TIFF is widely supported by scanning, faxing, word processing ...
or
.pdf Portable Document Format (PDF), standardized as ISO 32000, is a file format developed by Adobe in 1992 to present documents, including text formatting and images, in a manner independent of application software, hardware, and operating systems. ...
) for web or e-mail delivery. The fax traffic never passes through the VoIP infrastructure as a dial-up modem call and therefore arrives reliably even if T.38 is not properly supported at some points in the network.


Voice over IP

DID service has similar relevance for
Voice over Internet Protocol Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. The terms Internet t ...
(VoIP) communications. To allow
public switched telephone network The public switched telephone network (PSTN) provides Communications infrastructure, infrastructure and services for public Telecommunications, telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that ...
(PSTN) users to directly reach users with VoIP phones, DID numbers are assigned to a communications gateway. The gateway connects the PSTN to the VoIP network, routing and translating calls between the two networks.


Sellers

In countries with multiple competing local providers, DID services can be purchased in bulk from a
Competitive Local Exchange Carrier A competitive local exchange carrier (CLEC), in the United States and Canada, is a telecommunications provider company (sometimes called a " carrier") competing with other, already established carriers, generally the incumbent local exchange carrie ...
(CLEC). For voice over IP resellers, some specialized CLECs (for local numbers) or interexchange carriers (for toll-free numbers) will deliver blocks of direct inward dial calls already converted to
Session Initiation Protocol The Session Initiation Protocol (SIP) is a signaling protocol used for initiating, maintaining, and terminating communication sessions that include voice, video and messaging applications. SIP is used in Internet telephony, in private IP telepho ...
(SIP) or common VoIP formats. The individual VoIP provider need only obtain an inventory of local or freephone numbers from VoIP-aware carriers in various regions, import them in bulk to an
IP PBX An IP PBX ("Internet Protocol private branch exchange") is a system that connects telephone extensions to the public switched telephone network (PSTN) and provides internal communication for a business. An IP PBX is a PBX system with IP connectivi ...
and issue them individually to end users. International DID numbers can be purchased in bulk from international providers. UK geographic DID numbers can often be obtained for free and can be terminated over SIP. A few US DIDs are available without monthly charges from vendors like IPKall (discontinued in 2016), but at the expense of the caller paying for a call to some expensive, rural location. The majority of vendors charge a nominal amount per number per month (as little as $1/month in small quantities) and then bill per-minute or per number of channels which can be simultaneously in use. For the caller, these numbers can be assigned to locations which are a local call.


Direct outward dialing

The corresponding service to DID for outgoing calls from a PBX to the central office exchange is called ''direct outward dialing'' (DOD) or ''Direct Dial Central Office'' (DDCO). This service is often combined with DID service and allows direct dialing of global telephone numbers by every extension covered by the service without the assistance of an operator. The
caller line identification 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 ...
(CLI) or ''caller-id'' of extensions for outgoing calls is often set to the extension DID number, but may be the organization's central switch board number.


See also

*
Dialed Number Identification Service Dialed Number Identification Service (DNIS) is a service offered by telecommunications companies to corporate clients which identifies the originally dialed telephone number of an inbound call. The client may use this information for call routing to ...
*
Dial plan In telecommunication, a dial plan (or dialing plan) establishes the permitted sequences of digits dialed by telephone subscriber and the manner in which a telephone switch interprets these digits within the definitions of the prevailing telephone nu ...
*
Voice over IP Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for the delivery of speech, voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. The terms In ...


References

{{reflist Telephone exchanges