# Connect Retell AI with SIP trunks

> Send calls to a Retell AI voice agent and let it make outbound calls on your numbers.

- URL: https://kb.voipcloud.online/teams-and-sip/sip-trunking/retell-ai/
- Section: Microsoft Teams and SIP trunking › SIP trunking
- Type: guide
- Applies to: All countries
- Last reviewed: 2026-11-09

This guide connects a Retell AI voice agent to your PBX, so it can answer calls to your number and make outbound calls with your caller ID. Retell AI needs two SIP trunks in the PBX: one for inbound calls and one for outbound calls.

For help with Retell AI itself, such as building agents, contact Retell AI.

## Before you start

- Order **two** trunk licences and enough channels: go to **Services** › **SIP trunking**, set the number of **Channels** (one per simultaneous call), click **Add licence** twice, tick **Agree to service terms and conditions** and click **Submit order**.
- Have a Retell AI account with at least one agent.

## 1. Create the inbound trunk in the PBX

This trunk sends incoming calls to Retell AI.

1. Go to the **PBX** tab. Drag **SIP trunk** from the **Accept, make and divert calls** group onto the canvas and click the gear icon.
2. Enter a **Name**, such as "Retell inbound", and set the **Number of channels**.
3. For **Mode**, choose **IP authentication**.
4. In **SIP URI address**, enter your phone number followed by Retell AI's SIP server, for example `61XXXXXXXXX@sip.retellai.com:5060`. Use the number in international format without spaces. Check Retell AI's own SIP trunking help for its current server address.
5. Click **Save**.
6. Connect the [Inbound number](https://kb.voipcloud.online/pbx/receive-calls/inbound-number/) for that phone number to the SIP trunk. You can also connect it from a voice menu or time condition, so only some calls go to the agent.

## 2. Create the outbound trunk in the PBX

This trunk lets Retell AI sign in to the PBX and make calls.

1. Drag another **SIP trunk** onto the canvas and click the gear icon.
2. Enter a **Name**, such as "Retell outbound", and set the **Number of channels**.
3. For **Mode**, choose **SIP registration**.
4. Choose a **SIP trunk call plan**. Without one, outgoing calls won't work.
5. Choose the **Caller ID** for outgoing calls.
6. Click **Save**, then click **Apply configuration**.

Open the outbound trunk's settings again and note the **SIP credentials**: **Domain**, **Username** and **Password**. Also note the **Transport protocol**: TCP, or TLS if **TLS encryption** is turned on in **Settings** › **General**.

## 3. Add your number in Retell AI

**Note:** Menus can change between versions. If a label is different, check the vendor's own help.

1. In Retell AI, click **Phone Numbers**, then click **+** and choose **Connect to your number via SIP trunking**.

   ![Retell AI Phone Numbers page with the Connect to your number via SIP trunking option](https://kb.voipcloud.online/images/teams-and-sip/sip-trunking/retell-ai-connect-sip-trunking.webp)

2. Fill in the form:

 | Retell AI field | Enter |
 | --- | --- |
 | **Phone Number** | Your phone number, the same one as in the inbound trunk's SIP URI. |
 | **Termination URI** | The outbound trunk's **Domain**, including the port. |
 | **SIP Trunk User Name** | The outbound trunk's **Username**. |
 | **SIP Trunk Password** | The outbound trunk's **Password**. |
 | **Nickname** | Optional. |

3. Click **Save**.

![Connect to your number via SIP trunking form in Retell AI](https://kb.voipcloud.online/images/teams-and-sip/sip-trunking/retell-ai-sip-trunk-form.webp)

You can also add the number twice: once without a username and password for inbound calls, and once with them for outbound calls.

## 4. Assign your agents

1. Click the number in **Phone Numbers**.
2. Choose your agent under **Inbound call agent** for calls the PBX sends to Retell AI.
3. Choose your agent under **Outbound call agent** for calls the agent makes.

![Retell AI phone number settings with the Inbound call agent list open](https://kb.voipcloud.online/images/teams-and-sip/sip-trunking/retell-ai-inbound-agent.webp)

## Check it works

1. Call your number from a mobile. The Retell AI agent should answer.
2. Start an outbound call from Retell AI to a mobile. The mobile should show the caller ID you chose.

The outbound trunk can show **Offline** in the PBX. That's expected with Retell AI: outbound calls still work.

## Next steps

- [SIP trunk](https://kb.voipcloud.online/pbx/accept-make-divert/sip-trunk/) for every setting, including the **No answer** and **Failed** paths if the agent doesn't pick up.
- Route only after-hours calls to the agent with [Time conditions](https://kb.voipcloud.online/pbx/conditions/time-conditions/).