DID stands for direct inward dialling.
It's a feature a telephone numbers to be associated with one or more telephone lines.
DID allows a personal number to be assigned to each employee without having to have a separate physical phone line. This allows telephone traffic to be split up and managed more easily. It also saves you money.
So if your company had 12 employees and each employee had a separate telephone number, your company could rent 4 physical trunk-lines from the telephone company that would allow 4 phone calls to take place simultaneously.
The other employees would have to wait for an available line (and anyone dialling into the system) while the 4 lines are in use. They would either get a busy signal or be directed to voicemail. Of course, we could monitor your usage to prevent people from getting a busy signal. So, in essence, with analytics, we could determine how many actual lines have to be purchased.
DID can be used for fax and voice transmissions.