Direct Inward Dialing (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 dialled is also given, so that the PBX can decide which person in the office to route the call to. A Private Branch eXchange (also called PBX or Private Business eXchange) is a telephone exchange that is owned by a private business, as opposed to one owned by a common carrier or by a telephone company. ... A telephone number is a string of decimal digits that uniquely indicates the network termination point. ...

Developed by AT&T, this feature enables companies to have fewer lines than extensions, while still having a unique number for each extension, callable from outside the company. AT&T Inc. ... In telecommunication, the term trunk has the following meanings: In a communications network, a single transmission channel between two points that are switching centers or nodes, or both. ...

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. A telephone operator at work on a private switchboard A telephone operator is either a person who provides assistance to a telephone caller, usually in the placing of operator assisted telephone calls such as calls from a pay phone, collect calls (called reversed-charge calls in the UK), calls which... An automated attendant system allows the caller to be automatically be transferred to a users extension without the intervention of a receptionist. ...

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.

DID (DDI) in the new VOIP World

When you buy a phone line from Vonage or some other phone service provider, offering phone service over broadband, the number that they provide to you, in technical terms is a DID number. This is the number that they have assigned to you to connect you to the old PSTN Networks around the world. Any service provider who wants to offer a phone service over ip address, needs to buy DID number from his CLEC or any other large service provider like Level 3 in the United States or go to a consortium to buy these numbers like didXchange - didx . [1] - Worlds First DID/DDI (Direct Inward Dialing Numbers) exchange. ...

If you are using a IP PBX like Asterisk and you want to connect yourself to PSTN so people can call your office, you can either 1) buy an Analog or E1/T1 card from Digium, or 2) buy DID number from a service provider like virtualphoneline.com, broadvoice.com or voicepulse.com that will then come to your IPPBX as a real phone line and then you can use as your phone number, and route it to your ivr or direct extension. You can get a FREE UK DDI from IPStar.us or buy commercial DID's from service providers like Virtual Phone Line Service.

External links

• Castelle DID Information

• Direct Inward Dialing Number (Also known as DID or DDI)

• Did number Exchange. For xchanging, buying and selling Did numbers in wholeslae, using SIP and iax2.

References

• Castelle