Direct Inward Dialing (DID, also called 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'.
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) 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.
US DID numbers can be purchased in bulk from a
Competitive Local Exchange Carrier.
International DID numbers can be purchased in bulk from international DID providers like:
DID World Wide (60 countries covered)
Voxbone (40 countries covered)
There are also a number of DID resellers offering DID numbers for individuals and small enterprises:
DID eXchange
Ring WorldWide call forwarding service
Virtualphoneline
PimpMyNumber (UK Numbering Options)