Direct Inward Dialing (DID) is a service of a local phone company (or local exchange carrier) that provides a block of telephone numbers for calling into a company's private branch exchange (PBX) system. Using DID via SIP Trunk, a company can offer its employees individual phone numbers for each person or workstation within the company without requiring a physical line into the PBX for each possible connection.

The DID system can be used for fax and voice mail as well as for live voice connections. Compared to regular PBX service, DID on IPPBX saves the cost of a switchboard operator, calls go through faster, and callers feel they are calling a person rather than a company. Its purpose is to allow a company to assign a personal number to each employee, without requiring a separate phone line for each. That way, telephony traffic can be split up and managed more easily.