Direct inward dialing (DID), 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 numbers 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.
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This system is also used by fax servers. 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.
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 gateway 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.
In the United States, DID numbers and services can be purchased in bulk from a Competitive Local Exchange Carrier (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.
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 (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.