Glossary
DID number
An external phone number assigned directly to a specific extension or user, so callers reach that person without going through reception.
also known as: Direct Inward Dial, DDI, Direct Inward Dialling
A DID (Direct Inward Dial) number - also called DDI in the UK - is an external phone number assigned directly to a specific extension or user. Callers dial the DID and reach the person directly, without going through reception or an IVR.
Common uses
- Per-staff DID - every employee has their own external number for their direct line.
- Functional DID - sales, support, accounts, bookings each have their own DID feeding their queue.
- Multi-site DID - each office has its own main number that routes to that site’s reception or queue.
- Marketing DID - campaign-specific numbers (Google Ads, billboards, radio) to measure source.
- Department fallback - if an extension is busy, calls to its DID can fall back to the team queue.
How DIDs work
The SIP trunk provider holds a block of numbers (e.g. 100 sequential numbers) and routes any inbound call on those numbers to your 3CX system. Your 3CX system reads the dialled DID and routes the call to the matching extension or queue.
Configuration is simple:
- DID 03 1234 5601 → extension 5601 (John in sales)
- DID 03 1234 5602 → extension 5602 (Jane in sales)
- DID 1300 SALES → sales queue (all sales agents)
DID and number portability
DIDs are portable like any other Australian phone number. When moving carriers, all assigned DIDs port together. The per-extension assignment stays the same; only the underlying carrier changes.
DID block sizing
For new deployments we typically provision DID blocks in increments of 10, 20 or 100 numbers, depending on user count and growth expectations.
