# DID Number Overview

A DID number is a dedicated phone number that carriers point at your phone system so callers can reach a specific destination directly. This page explains what DID numbers are and the two ways Cloud Voice puts them to work.

## What is a DID number?

Direct Inward Dialling (DID), sometimes written as Direct Dial-in (DDI), is a service sold by phone carriers. Rather than handing out a single number, the carrier allocates a block (range) of numbers to one trunk. DID numbers carry an additional cost, so reach out to your trunk provider to buy the numbers you need.

:::note[What is a trunk?]
A trunk is the connection between Cloud Voice and your carrier's network that carries calls in and out. Carriers typically sell phone numbers in blocks tied to a trunk rather than one number at a time, which is why DID numbers are managed against a trunk.
:::

## How Cloud Voice uses DID numbers

In Cloud Voice, you apply DID numbers in one of two places, an inbound route or a trunk, and each placement serves a different goal.

### On an inbound route

Assigning DID numbers to inbound routes lets you steer calls based on the number that was dialed:

- Separate calls by purpose. Give each department or business line its own number so that calls for customer service, sales, and other teams follow their own path through the system.
- Reach people directly. Hand individual DID numbers to specific employees, and callers can dial straight through to an extension without going through a menu or receptionist.

:::tip
Routing by the dialed number lets you change how one team's calls are handled without touching any other team's setup. It also makes it easy to publish a distinct, memorable number for each business line.
:::

For step-by-step configuration, see [Route Inbound Calls based on DID Numbers](/pbx/administrator-guide/route-inbound-calls-based-on-did-numbers/).

### On a trunk

DID numbers configured at the trunk level cover two needs:

- SIP register trunks. When your provider issues DID numbers that differ from the trunk's SIP authentication name, add every provided number to the trunk.
- Call identification. Pair each DID number with a DID name so you can tell which number a caller dialed.

:::note
SIP (Session Initiation Protocol) is the signaling method Cloud Voice uses to set up and end calls with your provider. A SIP register trunk logs in to the provider using an authentication name, much like a username. Your provider may be referred to as an ITSP (Internet Telephony Service Provider).
:::

:::caution
On a SIP register trunk, if the provider's DID numbers do not match the SIP authentication name, you must add each DID number to the trunk. Inbound calls over that trunk will fail until you do.
:::

For step-by-step configuration, see [Configure DID Numbers on a Trunk](/pbx/administrator-guide/configure-did-numbers-on-a-trunk/).