Direct Inward Dialing (DID), also called Direct Dial-In (DDI) in Europe, is a feature offered by telephone companies for use with their customers' PBX systems, whereby the telephone company (telco) allocates a range of numbers all connected to their customer's PBX. As calls are presented to the PBX, the number that the caller dialed is also given, so the PBX can route the call to the desired person or bureau within the organization.
Developed by AT&T in the 1960s, patterned upon the earlier IKZ service of the Deutsche Bundespost, this feature enables companies to have fewer lines than extensions, while still having a unique number for each extension, callable from outside the company.
By way of example, each extension of the PBX system may be assigned a seven-digit external telephone number, with a fixed four- or five-digit prefix. Someone who knows the internal extension of his/her correspondent can dial the seven-digit number and be connected directly to the person called, bypassing the operator or PBX auto-attendant.
This system is also used by fax servers. Instead of an exchange at the end of the 234 000 line, a computer running fax server software and fax modem cards uses the last three digits to identify the recipient of the fax. This allows 1000 people to have their own individual fax numbers, even though there is only one 'fax machine'.
The corresponding trunk for outgoing calls from PBX to central exchange is called Direct Dial Central Office or DDCO.