Source : Free On-Line Dictionary of Computing

Direct Inward Dialing
     
         (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://www.ora.com/reference/dictionary/terms/D/Direct_Inward_Dialing.htm)}.
     
        (1997-06-29)