Glossary Phone numbers
What is DID?
Also called direct inward dialing, direct dial in, ddi, direct inward dial
A DID, Direct Inward Dialing number, is a public phone number routed straight to a particular extension, person or department inside a phone system. It lets an organisation give out many distinct numbers without buying a separate line for each one, because they all arrive over shared capacity and are sorted out by the phone system.
Why DIDs are cheap now
On the old network, a number that rang directly usually meant a physical line. On a modern system the number is a routing instruction, so the marginal cost of another one is small. IZT charges $1.00 per number per month. That is why it is now normal to give every employee a direct number, and to spin up separate numbers for campaigns, locations or departments purely to see which ones ring.
Uses beyond the obvious
Because DIDs are cheap and independently routable, they are useful as measurement. A distinct number on each advertisement, landing page or vehicle livery tells you exactly which one generated the call, without asking the caller. The same trick separates support from sales at the network level so the routing never depends on the caller pressing the right key.
DIDs and E911 obligations
Every DID needs a registered street address so that an emergency call from it reaches the right dispatcher. This is easy to forget when numbers are cheap and quick to add, and it is the most common compliance gap in a fast-growing deployment. If you provision numbers for a new office, a remote worker or a temporary campaign, the address has to be set and kept accurate. A number with a stale address is worse than no number, because it sends responders somewhere confidently wrong.