# Use number lists for load balancing

**Scenario:** A global service provider needs to route incoming calls based on the caller's country code (CLI prefix),
while using a single DID number.

**Solution:** To manage this, the provider assigns specific number lists to trunks within a trunk group. The system uses
the CLI prefix to match the call to the correct number list and selects the appropriate trunk for routing.

For example:

- Calls whose calling numbers begin with `1` are routed to a US-based trunk. Prefix `1` covers all countries in the North American Numbering Plan, not only the United States.
- Calls whose calling numbers begin with `44` are routed to a UK-based trunk.

This setup enables efficient load balancing and region-specific routing while maintaining a single DID.

## Step 1: Create Number Lists

The prefixes `1` and `44` match only calling numbers in E.164 format without a leading `+`. For a SIP trunk, this
example requires **CLI Format** set to **E.164** and no **CLI Prefix**.

1. Go to **Voice > Inbound Trunks** and open the **Number Lists** tab.
2. Click **Create New** and enter a **Friendly name** (e.g., *US Source Filter*).
3. Set the **Mode** to **Prefix match**.
4. Set the **Default action** to **Reject call**.
5. Click **Create**.

[![Creating a Number List for Load Balancing - US](../../../../_images/fig1e4.png)](../../../../_images/fig1e4.png)


**Fig. 1.** Creating a Number List for Load Balancing (US)

6. Click the **Actions** button next to the **US Source Filter** Number List.
7. Select **Manage numbers**.
8. Click **Add Numbers**.
9. Enter **1** and set the action to **Allow call**.
10. Click **Submit**.

[![Adding US Prefix](../../../../_images/fig2e4.png)](../../../../_images/fig2e4.png)


**Fig. 2.** Adding 1 Prefix for US

11. Repeat the steps above to create a second list named *UK Source Filter* and add prefix **44**.

[![UK and US Source Filter Number Lists](../../../../_images/fig3e4.png)](../../../../_images/fig3e4.png)


**Fig. 3.** UK and US Source Filter Number Lists

## Step 2: Assign Number Lists to trunks

This step uses two existing SIP trunks, **US SIP Trunk** and **UK SIP Trunk**. If you need to create them first, see
[Create a SIP trunk](../../sip-trunks/how-to-guides/create-sip-trunk.html).

To assign the number lists to trunks, see [Assign a Number List to a SIP
Trunk](../how-to-guides/configure-number-lists.html#inbound-number-lists-assign-sip) or follow these steps:

1. Go to **Voice > Inbound Trunks**.
2. Click the name of the trunk, and then click **Actions > Edit** on the trunk page.
3. Open the **Number Translations** tab and locate **CLI Number List**.
4. Select the **Number List** for the trunk and click **Submit** to save the changes. Assign **US Source Filter** to **US SIP Trunk**.
5. Repeat the steps above to assign **UK Source Filter** to **UK SIP Trunk**.

[![Edit Inbound SIP Trunk Number Translations tab with US Source Filter selected as the CLI Number List](../../../../_images/fig4e4.webp)](../../../../_images/fig4e4.webp)


**Fig. 4.** Assign the Number Lists to Trunks

## Step 3: Create a Trunk Group

To create a trunk group, see [Create a Trunk Group](../../trunk-groups/how-to-guides/create-trunk-group.html) or follow
these steps:

1. Go to **Voice > Inbound Trunks**.
2. Click **Create New** and select **Trunk Group**.
3. Enter the trunk group **Name**, for example *Load Balancing Trunk Group*.
4. In **Trunks**, add **US SIP Trunk** and **UK SIP Trunk**, then click **Create**.

[![Create Inbound Trunk Group form named Load Balancing Trunk Group with UK SIP Trunk and US SIP Trunk](../../../../_images/fig5e4.webp)](../../../../_images/fig5e4.webp)


**Fig. 5.** Create a Trunk Group

## Step 4: Assign the Trunk Group to a DID number

1. Navigate to **Phone Numbers > My Numbers**.
2. Select the desired **DID number** and click on the voice trunk field. If the DID has no trunks assigned, the trunk field will display `Voice: none`.
3. Assign the **Load Balancing Trunk Group** and click **Confirm**.

For more details, see [Assign a voice
trunk](../../../../phone-numbers/my-numbers/how-to-guides/assign-voice-trunk.html#user-panel-assign-trunk).

[![Assigning the Trunk Group to a DID Number.](../../../../_images/fig6e4.png)](../../../../_images/fig6e4.png)


**Fig. 6.** Assigning the Trunk Group to a DID Number

## Result

- Calls whose calling numbers begin with `1` are routed to **US SIP Trunk**.
- Calls whose calling numbers begin with `44` are routed to **UK SIP Trunk**.
- All other calls are rejected by default.

This setup allows a single DID number to intelligently route incoming traffic to country-specific trunks, improving
routing efficiency and regional control.

Tip

Scale the trunk group to include as many trunks and number lists as needed to achieve traffic load balancing in expected
countries or regions.

## How it works

Each trunk in the group accepts only the calls that its **Number List** allows. When a trunk's **Number List** rejects a
call, the trunk group tries the next trunk. This works because **403 Forbidden**, the response for calls rejected by a
Number List, is selected by default in the SIP trunks' **Re-routing Disconnect Codes**. Make sure **403 Forbidden** is
still selected on both trunks, because existing trunks may use different settings.

In this example, the Number Lists act as filters. They decide which trunk can accept a caller, so the setup mainly
demonstrates prefix-based routing. **Priority** and **Weight** decide how the group chooses among trunks that can accept
the same caller. For how trunks in a group are selected, see [Service failover and load
balancing](../../sip-trunks/call-handling-and-delivery/service-failover-and-load-balancing.html#inbound-sip-failover-load-balancing).

## Related resources

- [Number lists](../index.html) - Understand how Number Lists match calls and
  allow or reject them.
- [Configure number lists](../how-to-guides/configure-number-lists.html) -
  Create a Number List, add numbers or prefixes, and assign it to a trunk.
- [Trunk groups](../../trunk-groups/index.html) - Understand how trunks in a
  group are selected and re-routed.
- [Service failover and load balancing](../../sip-trunks/call-handling-and-delivery/service-failover-and-load-balancing.html#inbound-sip-failover-load-balancing) - Review routing attempts and
  re-routing behavior.
- [Number List settings reference](../number-list-settings-reference.html) -
  Look up field definitions, defaults, values, and limits.

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