DID Direct inward dialing (DID), is a specific feature offered by telephone services providers for implementation with their associated PBX systems. When DID is evolved, it means that telephone company ensures one or several so called trunk lines to the customer in order to get connectivity between PBX and designated amount of extensions (telephone numbers) sitting on this line (or group of lines) and process all phone calls to these numbers through the trunk. Within DID calls are delivered to the PBX, the dialed destination number(DNIS) is provided either, usually some digits of it (e.g., last three digits), therefore PBX can re-route the call straightly to the needed extension within the building, organization, premises or structure without the need for an live agent or attendant. DID service ensures direct inward call routing to every extension while upholding only a few amount of subscription phone lines in order to fill all needs of simultaneous inbound inquires.

Historically DID circuits appear to be analog since its introduction at the mid-20 century. Analog types of DID trunks had to be served and powered by the existing premises hardware. The centric configured office equipment figures out the power status of a line and disconnects service if the circuit is not empowered. This DID methodology is the opposite arrangement compared with traditional POTS (Plain Old Telephone Lines) which are normally powered from the central console. Modern appliances commonly deliver DIDs on a PRI (Primary Rate Interface) circuit.

DID service has the same performance within VoIP telecommunications. To approach users with VoIP phones, DID numbers are designated to a specific gateway connected between the trunk to PSTN and VoIP network. The digital gateway re-routes and processes phone calls between these two networks for the VoIP user. Calls initiated in the VoIP network will reach a user on the PSTN end as started from one of the specifically endorsed DID number. In North America DID numbers and services can be bought from a Local Excahge Carriers (CLECs). International DID numbers can be acquired from international providers. UK DID numbers can be received for free, used for SIP termination and bring up revenue for incoming voice and data traffic

The associated service to DID for forwarding phone calls from a PBX to the central office exchange is labeled as DOD (Direct Outward Dialing). Normally this service is combined with DID and enables direct dialing of international telephone numbers by each extension allowed by the service without the help of a live operator. Sometimes CLI (Caller identification) number of extension for outgoing calls is often attached to the extension DID number, but in some cases is the organization’s central switch board access number. So DID originally was created as a method to re-use a limited amount of physical phone lines to manage calls to various published numbers. In a business with DID, the phone company implements DID to identify the number they are about to connect to the business’s PBX. Originally this was executed by buttoning the last 3 or 4 digits of the number being dialed prior connecting the specific number. The PBX would use these DID digits to switch the call to the proper addresses.