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.
DID service has similar relevance for Voice over Internet Protocol (VoIP) communications. To allow public switched telephone network (PSTN) users to directly reach users with VoIP phones, DID numbers are assigned to a communications gateway. The gateway connects the PSTN to the VoIP network, routing and translating calls between the two networks.