The term predates the technology that carries it. It was specified for ISDN, the digital telephone network that came before IP carried calls. It survives in products that have never used a physical line, which is why it sounds older than the SIP trunk it now runs over.

## Where does the term come from?

An ISDN supplementary service, which is why it sounds older than SIP.

The formal reference is [ITU-T I.251.1](https://www.itu.int/ITU-T/recommendations/rec.aspx?rec=I.251.1&lang=en), from the standards arm of the International Telecommunication Union, the UN agency that administers the numbering plan. It is titled Direct-dialling-In. It was published in 1992, under number identification supplementary services in the ISDN series. The service was specified for a world of physical lines. The vocabulary outlived the technology.

What replaced the mechanism is a number pointed at a SIP trunk rather than at a block of copper. The service the caller experiences is identical. The term persists in products that have never touched ISDN.

## What is the difference between a DID and an extension?

One is dialable from outside, the other is not.

| Thing                      | Who can reach it                                | What it costs you               |
| -------------------------- | ----------------------------------------------- | ------------------------------- |
| DID number                 | Anyone, from the public network                 | A number, per month             |
| Extension                  | Callers already inside the phone system         | Nothing external                |
| SIP trunk                  | It carries the calls rather than receiving them | Capacity, by simultaneous calls |
| Simultaneous-call capacity | Not a destination at all                        | The limit on concurrent calls   |

The pairs that get conflated are the last two rows against the first. Holding fifty DID numbers does not mean fifty people can be on the phone at once, because concurrency is a property of the trunk. Nor does one trunk with capacity for fifty calls give anyone a number to dial.

## How does a call actually arrive?

Public network, then your provider, then your trunk, then your own routing.

A caller dials the public number. The originating network routes it to whoever holds that number, which is your provider. Your provider delivers it over the SIP trunk you have configured. Your phone system then decides which endpoint rings, using its own extension plan.

The last step is yours rather than the provider's. A provider hands the call to your system with the dialed number attached. Mapping that number to a person, a queue or a menu happens in your phone system rather than in the carrier's.

## Does a DID decide what my outbound calls show?

No. Inbound routing and outbound identity are separate.

The number a call arrives on and the number your outgoing calls present are configured in different places. They can differ freely. A business can take calls on fifty DIDs and present one main number on everything it dials out.

That separation is why the two questions have different answers on any platform. [CNAM against caller ID](/explained/numbers/what-is-cnam-and-how-is-it-different-from-caller-id) covers what the person you call actually sees.

## How do I set one up on Bird?

Buy a number with voice capability, then point its inbound route at where you want the calls.

Check `capabilities` before you buy. `GET /v1/numbers/available` accepts `capabilities=voice` and returns only numbers that support it. `POST /v1/numbers/orders` then takes the exact value the search returned.

Every number starts with its route set to `reject`, which refuses calls rather than leaving the route unset. So configuring the route is a required step, not a default to override.

Changing that route is a dashboard and command-line operation rather than a public API one. [Receiving calls](/docs/guides/voice/receiving-calls) covers the routing options. [SIP trunks](/docs/guides/voice/sip-trunks) covers delivering the calls to your own phone system, which keeps its own extensions.

## Related resources

- [SMS numbers](/products/sms/numbers) (product)
- [Number types](/docs/guides/numbers/number-types) (docs)
- [What is a virtual phone number (VMN)?](/explained/numbers/what-is-a-virtual-phone-number) (answer)

[Get an implementation brief](/learn/workspace?topic=phone-numbers)