Direct Inward Dialing
From Wikipedia, the free encyclopedia
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 dialed is also given, so that the PBX can decide which person in the office to route the call to.
Developed by AT&T after the middle 20th century, 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'.
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.
[edit] DID (DDI) and VoIP aka "Call Through"
"DID" numbers have particular relevance for VoIP communications. In order for people connected to the traditional PSTN network to call people connected to VoIP networks, DID numbers from the PSTN network are obtained by the administrators of the VoIP network, and assigned to a gateway in the VoIP network. The gateway will then route calls incoming from the PSTN across the IP network to the appropriate VoIP user. Similarly, calls originating in the VoIP network will appear to users on the PSTN as originating from one of the assigned DID numbers.
DID numbers can be purchased in bulk from a Competitive Local Exchange Carrier. There are also a number of DID resellers offering DID numbers for individuals and small enterprises.
[edit] External links
- DID number eXchange. DIDX is an Online service for exchanging, buying and selling DID numbers in wholesale, using SIP, Iax2 and h323 protocols. DIDX is now the world's largest company to offer services of DID numbers with over 3200 telecom companies as its members and hosting over 1 million phone numbers on its switches.
- Phone 2 Net. Is an Online service Buying DID numbers / Sip Trunking from around the world on per channel bases.
- AQL - A wide choice of 2.5 million UK geographic DID numbers
- DID World Wide Offers online ordering of DIDs all over the globe forwarded to SIP, IAX, H323 or PSTN
- Virtual Phone Line Gives DID number / Phone number on SIP, IAX2, H323, Gizmo Project, FWD or PSTN Phone Number anywhere in the world, from almost anywhere in the world.
- Groovy TEL Gives a US, UK or France DID number / Phone number on MSN Messenger or Google Talk User can be anywhere in the world, to receive the call.
