NumberGlossaryDirect Inward Dialing DID
General telephony (widely used across North America and, as DDI, internationally) · also called DID number, direct dial-in, DDI, network inward dialing
A service feature that lets an outside caller reach a specific PBX extension directly, without an operator, by dialing an assigned public number.
Direct Inward Dialing (DID) is a telephone service feature that routes an inward-directed call to a specific extension behind a PBX (private branch exchange) without human intervention. The telephone company assigns a range of public numbers to the customer and passes the dialed digits to the PBX, which connects the caller straight to the intended extension rather than to a central switchboard or receptionist.
Practically, DID lets an organization give each employee, department, or line a unique dialable number while sharing a smaller pool of physical or virtual circuits. Outside North America — chiefly the UK, Europe, and Oceania — the same feature is commonly called Direct Dial-In (DDI). In modern VoIP systems, a "DID number" often refers to any individually addressable telephone number provisioned to a customer, even without a traditional on-premises PBX.
DID is a routing/service feature, not a physical line count. A common misunderstanding is that each DID number requires its own dedicated trunk or circuit; in fact a range of DID numbers is delivered over a smaller shared set of circuits, and the dialed digits determine which extension the call reaches.