direct inward dialing (DID)

Telephone company facility that allows an outside caller to connect directly to an inside extension of an office without the help of an attendant or operator. Used mainly in direct marketing, each DID line (such as a 800 or 900 number) has a dialed number identification service (DNIS) number attached to it. The firm's computer or PBX is programmed to recognize this number and to accordingly route the call to its specific extension. See also direct outward dialing.