We’re all used to Interactive Voice Response menus – when you call your bank or ISP, you’ve probably had to navigate at least one (but often up to four nested menus!). Listening closely for the required menu option, then picking the closest when none really align to your goal, and pressing the dreaded option to repeat the menu prompt all over again could be behind us.

ElevenLabs offers AI Voice Agents that support natural language processing and generation, and supports integration into phone systems using SIP services, which makes it perfect for integrating into 3CX. We’ll start with setting up your AI voice agent, and move onto configuring 3CX to handle calls in and out of the system.

Please note that this blog post contains affiliate links – helps me keep the lights on!

Understanding the Players: 3CX and ElevenLabs AI Voice Agent

Before we get our hands dirty with configuration, let’s briefly introduce the two stars of our show:

3CX Phone System

For those unfamiliar, 3CX is a leading open-standard IP PBX that offers a comprehensive suite of unified communications features. It’s a software-based solution that allows businesses to manage their phone calls, video conferencing, live chat, and more, all from a single platform. Whether you’re a small startup or a large enterprise, 3CX provides flexibility, scalability, and cost-effectiveness. It can be deployed on-premise, in the cloud, or as a hosted solution, making it adaptable to various business needs. With features like auto-attendants, call queues, CRM integration, and mobile apps, 3CX empowers businesses to handle communication efficiently and professionally.

Note that for today’s project, you’ll need to be running the self-hosted or private-cloud version of 3CX, as it requires you setup a custom SIP provider which is unsupported in 3CX’s own cloud-hosted option.

ElevenLabs AI Voice Agents

ElevenLabs is at the forefront of AI speech technology, and their AI Voice Agents are truly revolutionary. These agents are designed to have natural, human-like conversations, understand context, and even perform actions based on user input. Unlike traditional IVRs that rely on rigid menu structures, ElevenLabs AI Voice Agents can engage callers dynamically, providing a much more fluid and satisfying experience. Their ability to synthesize high-quality, realistic voices adds another layer of authenticity, making interactions feel less robotic and more human. From customer support to information retrieval, the possibilities with ElevenLabs AI Voice Agents are virtually limitless.

Building Your Smart AI Voice Agent

This is the fun part – let’s create an ElevenLabs AI Voice Agent that will intelligently route your calls.

While writing this post, the kids are watching Monsters Inc, so naturally I’ll be using references to the characters and organization in the film.

1. Setting up your 3CX Phone System

You’ll need to be running an on-premises or private-cloud version of 3CX for this to work. You can get a free trial of 3CX Pro for 1 month. If you’re private-cloud hosting, I’d recommend spinning up a virtual machine in Vultr.

While I won’t cover the basic setup for 3CX, as 3CX has done a great job of covering this themselves.

Once you’re logged in to the Admin section of your 3CS system, we’re ready to get onto the first part.

Creating departments, users and call queues

Go to Users and create one for each person who’ll be answering the phone. I’ve created three users: James Sullivan, Mike Wazowski and Randal Boggs. You can optionally create departments but it’s not required to get you started.

Next, go to Call Handling and click + Add Queue. Set the name of the queue that the user should ask for and take note of the Virtual extension number as you’ll need this when you route your call from your AI Voice Agent. Go to the Users tab and add the users you created from the previous step as required. Save the queue, and repeat for each department you need.

My Queues are as follows:

Creating a custom SIP trunk and call routes

Let’s get 3CX ready to connect to ElevenLabs.

Go to Voice & Chat and click + Add Trunk. Set your country accordingly, the provider to Generic SIP Trunk (IP Based).

Once the trunk has been created, open the trunk, and on the General tab, wset the following options:
Name: ElevenLabs Trunk
Main Trunk Number: You can use any number here, as long as it’s populated
Registrar/Server: sip.rtc.elevenlabs.io

On the Options tab, set the following options:
Transport mode: TCP
Include diversion header support: Yes

You’ll be prompted to create an outbound rule for this trunk, set the following options:
Calls to numbers starting with prefix: Since ElevenLabs will only work with proper E164 phone numbers, I’m using phone numbers reserved for fictional use in my country. Is this the correct way to do it? Probably not. Does it work? Yes. My example is 61355501234
Calls to numbers with a length of: The length of your phone number
Prepend: +

If you’re using multiple agents, you could use a number range and tweak the prefix to add more options.

Now we’re done with the 3CX part, now to move onto ElevenLabs.

2. Creating a Basic AI Voice Agent

First things first, if you haven’t got one already, create yourself an account with ElevenLabs.

Login to your account and navigate to Conversational AI -> Agents, and click New Agent. I’d suggest starting with Business Agent and the type of business you’re building it for. Follow the prompts to create a starting point for your agent. I’ve named my agent Celia Mae, after the receptionist at Monsters Inc.

Once your agent is created, tweak the System Prompt to your liking, in terms of personality and tone. In the goal section, we’ll need to make sure your agent is prepared to transfer your caller to an extension on your phone system. Here’s my System prompt for Celia:

# Personality

You are Celia, a professional and efficient answering service representative for Monsters Incorporated. You are polite, clear, and helpful, ensuring all callers are directed to the correct department.

# Environment

You are answering phone calls for Monsters Incorporated. Callers are external and may be trying to reach Sales, Accounts, or Service departments.


# Tone

Your responses are bubbly, courteous, and friendly. You should use inclusive and welcoming language, but be brief. Avoid jargon. Speak at a moderate pace and enunciate clearly.

# Goal

Your primary goal is to efficiently direct incoming calls to the correct department within Monster Incorporated.

1.  Greeting: Answer the call promptly with a professional greeting, "Thank you for calling Monsters Incorporated, this is Celia, how may I direct your call?"
2.  Information Gathering: Clearly identify which department the caller needs to reach (Sales, Accounts, or Service). If the caller is unsure, ask clarifying questions to determine the nature of their inquiry.
5.  Confirmation: Confirm which department the caller has asked for politely. "So it was {{department}} you were after? Let me transfer you now." And transfer the call immediately.

# Guardrails

Do not provide information about company financials, internal policies, or employee information beyond publicly available directory information. Transfer calls directly. If a caller is abusive or uses inappropriate language, politely end the call.

# Tools

None

Next, scroll down to the Tools section and enable End Call and Transfer to number. Once enabled, click the settings gear icon next to Transfer to number.

Under the Human Transfer Rules, click Add Rule and set the Transfer type to SIP REFER.

Set the Number type to SIP URI, and set the SIP URI value to “sip:<queue extension number>@<3cx server address>”. It should look a bit like: “sip:811@my-ai-demo.3cx.com.au”. The queue extension number is the number you recorded while setting up the queue in 3CX.

Set the condition for the first department you’d like to transfer to: “If the caller wants to speak to the Accounts department”.

Click Add Rule again and repeat for every department you wish to add.

You might also want to add a condition for if the caller becomes agitated or asks to speak to a human.

We’re pretty much done setting up our agent. Feel free to customize your agent further, like the voice selection.

Don’t forget to save your agent!

3. Connecting ElevenLabs to 3CX

We’re now on the home stretch!

Navigate to Telephony -> Phone Numbers and click Import Number -> From SIP Trunk.

Set your Label to something logical, I called mine Monsters Inc Switchboard. Then set your phone number to the number you set in the Outbound rule in 3CX.

Set the Origination URI to “sip:sip.rtc.elevenlabs.io:5060;transport=tcp”, and Media Encryption to “Allowed”.

In the Outbound Configuration, set these options:
Address: <your 3cx server FQDN> ie: my-ai-demo.3cx.com.au
Transport Type: Auto
Media Encryption: Allowed

Save the SIP trunk, then set the agent for your newly created SIP trunk.

4. Finalization

Great, everything is now setup!

From one of your extensions, dial the phone number you setup for your AI Voice Agent. With any luck, you’ll hear an AI generated voice asking where you’d like your call directed. Ask it to put you through to accounts and see how it goes!

No doubt you’ll want to tweak your agent before going live, but once you’re happy with your agent, setup one of your inbound routes to forward to the external number for your agent.

Did you get stuck or need help?

It is a bit of a process to get setup, but understanding where to look when things go wrong can point you in the right direction.

Check your call logs in 3CX and double check your configuration.

Here’s some issues I found while doing the initial implementation:

My call never connects or rings out
Make sure you added the + in the prepend section in your outbound rule, and your numbers match in both your 3CX outbound rules and Phone Numbers in ElevenLabs.

My agent says the call transfer failed
Make sure Include diversion header support is enabled on your ElevenLabs SIP trunk. Also check the queue extension numbers match your Human Transfer Rules in your agent configuration.

My agent gives me an extension number and refuses to transfer me
Make sure you enabled Transfer to Number in the agent’s tools section, and in your System prompt told the agent they can transfer calls directly.

We provide telephony consulting services at my day job, so please get in touch if you need further help.

• ai
• ivr
• sip
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  2025-08-30 0