Welcome to the ZyXEL Networking Glossary - your definitive source for descriptions of networking terms and ZyXEL's product features.
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DDI (also called DID, Direct Inward Dial) is a feature that maps a public phone number to an extension number. DDI enables a caller to call an extension number without going through an operator. When people give out their contact number and say it's a “direct line”, often what they mean is that it's a DDI number.