Direct inward dialing

Direct inward dialing (DID), also called direct dial-in (DDI) in Europe and Oceania, is a telecommunication service offered by telephone companies to subscribers who operate a private branch exchange (PBX) system. The feature provides service for multiple telephone numbers 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.

For direct inward dialing 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 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 (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 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 (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.

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