DID stands for Direct Inward Dialing. In the classic telephone-company sense, it is a service that lets the public dial a number and land on a specific PBX extension without an operator. The serving central office seizes a DID trunk group into the PBX and outpulses the last digits of the called number. The PBX maps those digits onto an extension.
That mapping is the whole idea: many public numbers, fewer physical trunks. A business could publish 100 directory numbers, buy a 20-trunk DID group, and let the PBX fan calls onto extensions 1000-1099. The public numbers were (and still are) ordinary NANP line numbers: an NPA-NXX-XXXX in a rate center, formatted in E.164 as +1 followed by those ten digits.
What “DID” means on IP networks
In VoIP, SIP trunking, and CPaaS, DID has become a synonym for any inbound telephone number assigned to a customer and routed over SIP (or another IP signaling path) rather than a dedicated TDM trunk into an on-premises PBX. The number still lives in the PSTN numbering plan. The termination is a SIP URI, a WebRTC client, a queue, or an What is IVR? instead of an analog extension.
Providers provision local geographic numbers, toll-free 8XX numbers, and sometimes national or mobile numbers, all as “DIDs.” Inbound media follows inbound (origination) economics: the provider that holds the number receives the call from the PSTN and delivers it to the customer. The opposite direction is outbound / termination.
DID, trunks, extensions, and number types
| Term | What it names | Where it lives | Typical use |
|---|---|---|---|
| DID (classic) | A public number mapped through DID trunks onto a PBX extension | LEC central office plus premise PBX | Direct-dial a desk without a receptionist |
| DID (IP / virtual number) | A PSTN number routed to a SIP or cloud endpoint | Numbering inventory of a carrier or CPaaS | Inbound voice (and often SMS) to an app or cloud PBX |
| DID trunk group | The TDM or SIP path that carries many DIDs with fewer channels than numbers | Between the serving switch and the PBX / SBC | Concentration: more numbers than simultaneous calls |
| Extension | An internal PBX address, often 3-5 digits | Inside the PBX numbering plan | Reached from the PSTN only via a DID map, an What is IVR?, or an attendant |
| Local number | A geographic NANP number in a rate center | NPA-NXX in the LERG / NPAC | Local identity; subject to Number porting |
| Toll-free number | An 8XX number in the SMS/800 database | RespOrg record, not a rate center | Caller is not billed for the call in the US; non-geographic |
How an inbound DID call is routed
- The caller dials the public number. The originating network looks up the called number in the PSTN (and, if the number has been ported, in NPAC for the Location Routing Number). See Number porting.
- The call is delivered to the carrier that currently serves the number.
- That carrier sends the call to the customer over DID trunks (TDM digits) or SIP (Request-URI / To header containing the DID).
- The PBX, What is IVR?, hunt group, or queue selects an agent or device.
The same DID can often send and receive SMS or MMS, but messaging registration (10DLC, Toll-free texting) is a separate compliance path from voice DID routing.