Numbers & trunks
DIDAlso known as Direct Inward Dialing, DDI (Direct Dial-In), Virtual number
Definition
A DID (Direct Inward Dialing), called DDI (Direct Dial-In) in the UK and Europe, is a public phone number that a carrier routes to a customer’s phone system, where it rings a specific extension, IVR, queue or ring group directly. With SIP trunks, many DIDs can share one trunk without needing a physical line per number.
How DIDs work
The carrier owns a block of numbers from the national numbering plan and assigns some to your account. When someone dials one, the carrier sends a SIP INVITE over your SIP trunk with the dialled number in the Request-URI or To header, usually in E.164 format (for example +14155550123). Your PBX matches it against inbound routes and sends the call to the right destination.
- Geographic DIDs carry an area code and give a local presence.
- Toll-free DIDs are paid for by the receiver.
- Mobile numbers are SIM-based and reached through the mobile network rather than a SIP trunk.
DID vs DOD
DIDs are also useful for tracking: giving each campaign, branch or department its own number shows exactly which source generated each call in the call records.
A DID handles inbound calls; DOD handles outbound calls and the caller ID shown. The same number often does both, so customers can call back the number they saw. See SIP trunks, DIDs and DOD explained.
Mobile numbers as inbound lines
A SIM in a GSM gateway behaves like a DID for its mobile number: calls to it ring the gateway, which forwards them over SIP to an extension, IVR or group. GSMCalls routes inbound mobile calls to your PBX in the same way, with CDRs for each call; see the platform. Number porting and use of mobile numbers must follow the operator’s terms.
Frequently asked questions
What is the difference between DID and DDI?
None in practice. DID is the North American term and DDI the UK and European term for the same service.
Can I keep my existing number as a DID?
Usually, by porting it to a new carrier, subject to the rules of your country’s regulator.
Does each DID need its own line?
No. Many DIDs share the channels of one SIP trunk; the PBX routes by dialled number.