Direct Inward Dialing is a technology that provides each employee with a personal phone number without a company having to invest in dedicated physical lines.
DID makes having multiple phone numbers an economical choice and makes it much easier for clients to contact employees directly.
Without DID, employees can still be contacted separately on their extensions. Extensions are one feature that sets PBX apart from a mere VoIP service. However, clients have to go through the process of calling the main number, navigate to the dial-by-extension option, and dial the relevant extension.
In a traditional phone system, a separate physical line is needed for every phone number. With DID, all desired numbers must be reserved, but only one physical line is required.
Calls arriving at this line are routed based on VoIP user accounts - each personal phone number is associated with such account, making physical lines redundant.