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19. What is a DDI? |
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Direct Inward Dialling (DID, also called DDI in Europe) is a feature offered by telephone companies for use with their customers' PBX system, 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 dials is also given so that the PBX can decide to which person in the office to route the call.
Developed by AT&T decades ago, this feature enables companies to have fewer lines than extensions, while still having a unique number for each extension, accessible 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'.
When people give out their work number and say it's a "direct line", often what they mean is that it's a DID number. |
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