# Extension Overview

An extension is a short internal number, for example 1001, that identifies a user on your phone system. People dial an extension to reach a coworker directly, and each extension is what lets its owner place and answer calls. In a typical setup you assign one extension to every member of your organization.

## Extension types

Cloud Voice uses SIP extensions. SIP (Session Initiation Protocol) is the standard signaling protocol that phones use to set up, manage, and end calls over a network.

Before a SIP extension can make or take calls, you register it on an endpoint. An endpoint is any device that signs in with the extension's credentials. It can be:

- a physical IP phone (a desk phone that connects over your network), or
- a softphone (a calling app that runs on a computer or mobile device).

:::note
Registering means the device logs in to Cloud Voice using the extension's number and password. An extension can exist in the system, but until at least one device is registered to it, the extension cannot send or receive calls.
:::

:::tip
The same extension can be registered on more than one device at the same time, for example a desk phone plus the Cloud Voice App on a mobile. The online status (below) lists every device the extension is currently signed in on.
:::

To create your first extension and get a phone registered, see [Create a SIP Extension](/pbx/administrator-guide/create-a-sip-extension/) and [Set up a SIP Phone](/pbx/administrator-guide/set-up-a-sip-phone/).

## Online status

Online status shows you, at a glance, which phone endpoints and Cloud Voice App clients are currently available for an extension. Hover over any status icon to see more detail about where the extension is signed in.

:::caution
An extension can only ring the devices where it is currently registered. If a desk phone or app is missing its status icon, nothing is signed in there, so calls to that extension will not reach that device. Check the registration before assuming a call was simply missed.
:::

### Phone endpoints

![SIP registration status icon](/images/pbx/SIP.png) The extension is registered on a SIP phone and ready to use. Hover over the icon to see the IP addresses of every SIP phone the extension is registered on.

### Cloud Voice App clients

![Desktop client status icon](/images/pbx/linkus-desktop.png) The Cloud Voice App desktop client is ready to use. Hover over the icon to see details about each signed-in desktop client, including the device type (Windows or Mac) and its IP address.

![Mobile client status icon](/images/pbx/linkus-mobile.png) The Cloud Voice App mobile client is ready to use. Hover over the icon to see details about each signed-in mobile client, including the device type and its IP address.

![Web client status icon](/images/pbx/linkus-web.png) The Cloud Voice App web client is ready to use. Hover over the icon to see the IP addresses of the devices running the signed-in web clients.