Abbreviation for Direct Inward Dialling. DID is a service offered by telephone companies which allows the last three or four digits of a telephone number to be transmitted to the destination exchange, in our case CCA.
For example, a company could have 10 incoming lines, all with the number 123 000. If a caller dials 123 456, the call is sent to 123 000 (the company's exchange), and the digits 456 are transmitted. CCA then routes the call to extension 456. 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.
CCA can also run as a fax server, thereby using 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.