# Seize a Trunk to Call Out by BLF Key

A BLF (Busy Lamp Field) key turns a physical button on a desk phone into a live indicator for a chosen trunk. Once provisioned, the key shows whether the trunk is free or in use, and pressing it seizes the line so you can dial straight out over that trunk. This page walks through assigning the key through Auto Provisioning and pushing it to the phone.

:::note[What a trunk is]
A trunk is the connection between your phone system and the outside telephone network (your carrier or SIP provider). Monitoring a trunk with a BLF key lets a user see at a glance whether that outside line is free before grabbing it, and lets them place a call over that specific trunk with one button press.
:::

## Before you start

- The phone must already be tied to Cloud Voice through Auto Provisioning and assigned an extension. Auto Provisioning is the process that pushes settings to the phone automatically, so you do not have to key them in by hand. If you have not done that yet, follow one of these guides first:
  - [Auto Provision IP Phones Remotely via RPS Method](/pbx/administrator-guide/auto-provision-ip-phones-remotely-rps-method/)
  - [Auto Provision IP Phones Remotely with Proxy](/pbx/administrator-guide/auto-provision-ip-phones-remotely-with-proxy/)
- To place calls through the monitored trunk from the BLF key, the extension on the provisioned phone must be allowed to use that trunk for outbound calls.

:::caution[Check outbound permission first]
If the extension is not permitted to use the monitored trunk for outbound calls, the key can still show the trunk's status, but pressing it will not place a call. Confirm the extension's outbound trunk permission before you hand the phone to a user.
:::

## Step 1: Add a function key that monitors the trunk

1. Sign in to the PBX (Private Branch Exchange, your Cloud Voice phone system) web portal and go to **Extension and Trunk > Extension**, then open the extension that belongs to the phone for editing.
2. Select the **Function Keys** tab.
3. Set up a function key to track the trunk's status. Fill in the fields as follows:
   - **Type**: Choose **BLF**.
   - **Value**: Type the name of the trunk you want to watch. In the example below, this is `peer-trunk-66.41`.
   - **Label**: Optional. Whatever you enter here appears next to the key on the phone's display.

   The example below monitors the trunk named `peer-trunk-66.41`.

   
   ![Cloud Voice, BLF function key configured to monitor a trunk, with Type set to BLF and the trunk name entered as the value](/images/pbx/set-up-blf-key-monitor-trunk.png)

   :::caution[The Value must match the trunk name exactly]
   The **Value** field has to match the trunk's name as it is spelled in the PBX (for example, `peer-trunk-66.41`). A typo or the wrong name means the key has nothing valid to watch, and it shows up as an unlit LED on the phone.
   :::

   :::tip[Give the key a clear label]
   Setting the **Label** to something like the trunk's purpose or number makes it obvious which line the button controls, which helps when a phone has several function keys.
   :::
4. Click **Save**.

## Step 2: Push the change to the phone

The function key does not reach the phone until the phone reprovisions and pulls down its updated configuration.

1. Go to **Auto Provisioning > Phones** and click the refresh icon ![Reprovision](/images/pbx/refresh.png) next to the phone you just configured. A prompt asks whether you want to reprovision it.
2. Click **OK** in the pop-up window.

:::caution[Reprovisioning reloads the phone]
Reprovisioning re-applies the phone's full configuration, which can briefly interrupt it. Do this while the phone is idle rather than during a call.
:::

## What you get

Once the phone reprovisions, the BLF key reflects the trunk's state in real time through its LED (the small indicator light beside the key):

- **Green LED**: The trunk is being monitored and is currently idle.
- **Red LED**: The trunk is busy.
- **LED off**: The BLF key did not configure successfully.

Press the key to grab the trunk. You will hear a dial tone, after which you can dial the number you want to reach.

:::tip[If the LED stays off]
An unlit key means the configuration did not take. Recheck that the **Value** exactly matches the trunk name, then reprovision the phone again (Step 2).
:::