DID stands for – Direct Inward Dialing (or DDI, Direct Dialling Inward in Europe) is a feature offered by telephone companies for use with their customers’ PBX system, whereby the telephone company allocates a range of telephone numbers associated with one or more phone lines. DID allows a company to assign a personal number to each employee, without requiring a separate physical phone line for each to connect to the PBX. This way, telephony traffic can be split up and managed more easily.
For example, if an organisation has 25 employees and each employee has a separate telephone number or extension, within its physical location, the organisation can rent 10 physical trunk lines from the telephone company that will allow 10 phone calls to take place simultaneously. Others would have to wait for an available line and anyone dialling into the system while all 10 lines are in use would get either a busy signal or be directed to a voicemail system. A DID system can be used for fax and voice transmissions.
DID works similarly for VoIP communications. To allow PSTN users to directly reach VoIP users, DID numbers are assigned to a gateway. The gateway connects the PSTN (public switched telephone network) to the VoIP network, routing and translating calls between the two networks for the VoIP user. Calls from the PSTN will be directed to the VoIP user who holds the corresponding DID number. DID requires that you purchase an ISDN or digital line and ask the telephone company to assign a range of numbers. You will then need DID capable equipment at your premises which consists of BRI, E1 or T1 cards or Gateways.