Direct Inward Dialing
Encyclopedia
Direct inward dialing also called direct dial-in (DDI) in Europe and Oceania, is a feature offered by telephone companies for use with their customers' private branch exchange (PBX) systems. In DID service the telephone company
provides one or more trunk lines to the customer for connection to the customer's PBX and allocates a range of telephone number
s to this line (or group of lines) and forwards all calls to such numbers via the trunk. As calls are presented to the PBX, the dialed destination number (DNIS
) is transmitted, usually partially (e.g., last four digits), so that the PBX can route the call directly to the desired telephone extension
within the organization without the need for an operator or attendant. The service allows direct inward call routing to each extension while maintaining only a limited number of subscriber lines to satisfy the average concurrent usage of the customer.
Traditionally, DID circuits were analog. These types of DID trunks had to be powered by the customer premises equipment. The central office equipment detects the power state of the line and disables service if the circuit is not powered up. This is the reverse arrangement from standard plain old telephone service
(POTS) lines which are powered by the central office. Nowadays, it is far more common to deliver DIDs on a PRI
circuit.
In the United States the feature was developed by AT&T
in the 1960s, patterned upon the earlier IKZ service of the Deutsche Bundespost
.
DID service is usually combined with direct outward dialing (DOD) allowing PBX extensions direct outbound calling capability with identification of their DID number.
s. A telephone line is terminated in a computer running fax server software and fax modem cards. 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.
connected by a trunk to the public switched telephone network
(PSTN) and the VoIP network. The gateway routes and translates calls between the two networks for the VoIP user. Calls originating in the VoIP network will appear to users on the PSTN as originating from one of the assigned DID numbers.
(CLEC). International DID numbers can be purchased in bulk from international providers. UK Geographic DID numbers can be obtained for free, terminated over SIP and also generate a revenue for inbound traffic.
(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.
Telephone company
A telephone company is a service provider of telecommunications services such as telephony and data communications access. Many were at one time nationalized or state-regulated monopolies...
provides one or more trunk lines to the customer for connection to the customer's PBX and allocates a range of telephone number
Telephone number
A telephone number or phone number is a sequence of digits used to call from one telephone line to another in a public switched telephone network. When telephone numbers were invented, they were short — as few as one, two or three digits — and were given orally to a switchboard operator...
s to this line (or group of lines) and forwards all calls to such numbers via the trunk. As calls are presented to the PBX, the dialed destination number (DNIS
Dialed Number Identification Service
Dialed Number Identification Service is a service sold by telecommunications companies to corporate clients that lets them determine which telephone number was dialed by a customer. This is useful in determining how to answer an inbound call. The Telecommunications company sends a DNIS number to...
) is transmitted, usually partially (e.g., last four digits), so that the PBX can route the call directly to the desired telephone extension
Extension (telephone)
An extension telephone is an additional telephone wired to the same telephone line as another. In middle 20th century telephone jargon, the first telephone on a line was a "Main Station" and subsequent ones "Extensions". Such extension phones allow making or receiving calls in different rooms,...
within the organization without the need for an operator or attendant. The service allows direct inward call routing to each extension while maintaining only a limited number of subscriber lines to satisfy the average concurrent usage of the customer.
Traditionally, DID circuits were analog. These types of DID trunks had to be powered by the customer premises equipment. The central office equipment detects the power state of the line and disables service if the circuit is not powered up. This is the reverse arrangement from standard plain old telephone service
Plain old telephone service
Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....
(POTS) lines which are powered by the central office. Nowadays, it is far more common to deliver DIDs on a PRI
Primary rate interface
The Primary Rate Interface is a standardized telecommunications service level within the Integrated Services Digital Network specification for carrying multiple DS0 voice and data transmissions between a network and a user....
circuit.
In the United States the feature was developed by AT&T
AT&T
AT&T Inc. is an American multinational telecommunications corporation headquartered in Whitacre Tower, Dallas, Texas, United States. It is the largest provider of mobile telephony and fixed telephony in the United States, and is also a provider of broadband and subscription television services...
in the 1960s, patterned upon the earlier IKZ service of the Deutsche Bundespost
Deutsche Bundespost
The Deutsche Bundespost was created in 1947 as a successor to the Reichspost . Between 1947 and 1950 the enterprise was called Deutsche Post...
.
DID service is usually combined with direct outward dialing (DOD) allowing PBX extensions direct outbound calling capability with identification of their DID number.
Use in FAX services
This system is also used by fax serverFax server
A fax server is a system installed in a local area network server that allows computer users whose computers are attached to the LAN to send and receive fax messages...
s. A telephone line is terminated in a computer running fax server software and fax modem cards. 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.
DID in Voice over Internet Protocol communications
Direct inward dialing service has similar relevance for Voice over Internet Protocol (VoIP) communications. To reach users with VoIP phones, DID numbers are assigned to a communications gatewayGateway (telecommunications)
In telecommunications, the term gateway has the following meaning:*In a communications network, a network node equipped for interfacing with another network that uses different protocols....
connected by a trunk to the public switched telephone network
Public switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...
(PSTN) and the VoIP network. The gateway routes and translates calls between the two networks for the VoIP user. Calls originating in the VoIP network will appear to users on the PSTN as originating from one of the assigned DID numbers.
Sellers
In the United States, DID numbers and services can be purchased in bulk from a Competitive Local Exchange CarrierCompetitive local exchange carrier
A competitive local exchange carrier , in the United States, is a telecommunications provider company competing with other, already established carriers ....
(CLEC). International DID numbers can be purchased in bulk from international providers. UK Geographic DID numbers can be obtained for free, terminated over SIP and also generate a revenue for inbound traffic.
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 identificationCaller ID
Caller ID , also called calling line identification or calling number identification or Calling Line Identification Presentation , is a telephone service, available in analog and digital phone systems and most Voice over Internet Protocol applications, that transmits a caller's number to...
(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.