Acronym for Direct Dial In. Sometimes also called DID (Direct Inward Dialling).

DDI is the ability to call directly to an extension when calling in to the PBX through a trunk line. This features may be available for both ISDN or SIP trunks. With ISDN, this is available for Point to Point connections.

When the feature is not present, inbound calls to the PBX may be routed either to the switchboard or to an IVR application that allows callers to specify the desired extension.

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