Short for direct inward dialing (also known as direct dialing inward), a service of an LEC or local phone company that allows an organization to have numerous individual phone numbers for each person or workstation in its PBX system that run off of a small block of dedicated telephone numbers. DID allows the multiple lines to be connected to the PBX all at once without requiring each to have a physical line connecting to the PBX.
For example, if an organization has 25 employees and each employee has a separate telephone number, or extension, within its physical location, the organization can rent 10 physical trunk lines from the telephone company that will allow 10 phone calls to take place simultaneously. Others would have to wait for an available line and anyone dialing into the system while all 10 lines are in use would get either a busy signal or be channeled into a voice mail system. A DID system does not require a PBX operator and can be used for fax and voice transmissions.