Direct Inward Dialing is a technology that provides each employee with a standard ten digits personal phone number (besides his extension or internal PBX phone number) without a company having to invest in dedicated physical lines.
DID makes having multiple phone numbers an economical choice for a company and makes it easier and faster for its clients to contact employees directly without navigating the company's phone menu.
Without DID, employees can still be contacted separately on their PBX phone lines or extensions, which is what sets a PBX system apart from a mere VoIP service. However, clients have to go through the process of calling the main number and navigating the dial-by-extension or dial-by-name options. With DID they simply dial a standard phone number to reach an employee.
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 SIP user accounts - each personal phone number is associated with such account, making physical lines redundant. The caller ID of the caller is carried over in the routing process for the benefit of the employee or agent who receives the call.