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Direct inward dialing is a service available through local companies that provide blocks of telephone numbers for calling a company’s private branch exchange system. A company offers the customer’s individual phone numbers for each person using direct inward dialing within the company without a physical line into private branch exchange for each connection.
Compared to regular PBX service, direct inward dialing saves the cost of a switchboard operator. These calls go through faster and provide callers with the sense that they are calling a person rather than a company. The purpose of a direct inward dialing is to allow companies to assign personal numbers to every employee without a separate phone line for each so that telephony traffic can be split up and managed efficiently. Direct inward dialing service is used in voice over internet protocol communications.
Direct inward dialing is a feature offered by telephone companies for use with customers' private branch exchange systems. The telephone company provides trunk lines to customers for connection to customers' private branch exchange (PBX), as well as allocates telephone numbers to lines and forwards calls to numbers through trunks. When calls are presented to PBX, the dialed destination number is partially transmitted so that PBX routes calls directly to the desired telephone extensions within the organization without having to use operators. The service allows direct inward call routing to every extension while maintaining a limited number of subscriber lines.
In U.S., direct inward dialing was developed in 1960 by AT&T. Initially, the technology was analog in nature and had to be powered by equipment on the customer's premises.
Direct inward dialing is generally combined with direct outward dialing, permitting PBX extensions to direct outbound calling with the identification of the direct inward dialing number.
Direct inward dialing requires a purchase of a range of numbers. Direct inward dialing equipment is then required on the premises.
When callers dial a person in a company through public telephone number, calls are received by the end office switch in public telephone company where the public telephone operator connects the call to available incoming trunk lines between the telephone company switch and PBX switch. The system looks up the called number to determine which extension the call has to be connected to and connects the incoming trunk line to correct telephone extension.
The dialed inward number is assigned to the communication gateway connected by trunk to PSTN and VoIP network. The gateway then routes and translates calls between two networks for VoIP users. Calls originating in VoIP network appear to users on PSTN as originating from the assigned direct inward dialing numbers.