Direct Inward Dialing definition communications
(DID) A service offered by telephone companies which allows the last 3 or 4 digits of a phone number to be transmitted to the destination exchange
For example, a company could have 10 incoming lines, all with the number 234 000. If a caller dials 234 697, the call is sent to 234 000 (the company's exchange), and the digits 697 are transmitted. The company's exchange then routes the call to extension 697. This gives the impression of 1000 direct dial lines, whereas in fact there are only 10. Obviously, only 10 at a time can be used.
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'.
Dictionary of PC Hardware and Data Communications Terms (http://ora.com/reference/dictionary/terms/D/Direct_Inward_Dialing.htm).