Direct inward dialing (DID) is a telecommunication service that allows organizations to allocate a range of phone numbers for direct incoming calls to their PBX system, improving communication efficiency. The guide discusses the benefits of DID, such as enhanced customer experience and privacy, and provides detailed steps for acquiring numbers, integrating with a PBX system, and troubleshooting issues. It emphasizes best practices for maximizing DID benefits through regular audits and security measures.