Direct Inward Dialing (DID) is a service provided by telephone companies. With this service, the local exchange transfers the complete subscriber number, including the extension number, to the company's PBX.
The PBX uses the extension telephone number to connect the incoming call directly to the extension. DID service allows callers to dial the contact's telephone line directly, without a central switch. Direct Inward Dialing (DID) can be used for telephone connections, for fax and voice mail.