Connect Inbound Calls to a Voice AI Agent with Twilio and LiveKit

1.Introduction

Imagine every phone call to your business being answered instantly, not by a human, but by a smart Voice AI agent. These agents never sleep and always know what to say, transforming customer service, appointment booking, and information sharing. But how do you get a real phone call connected to a Voice AI agent in real time?

In this post, you’ll learn how to directly route real inbound phone calls from Twilio to a Voice AI agent using LiveKit’s SIP capabilities. We’ll walk through how Twilio manages incoming calls, how LiveKit bridges them into real-time audio rooms, and how to connect it so your voice AI agent can start talking to callers instantly.

By the end of this guide, you’ll have a clear workflow and practical setup for integrating Voice AI into your phone system using industry-standard tools.

2.Understanding SIP Trunking

Before you can connect a phone call to a Voice AI agent, it’s important to understand how voice communication moves from the traditional phone network into your application. This is where SIP trunking comes in.

Session Initiation Protocol (SIP) trunking is a modern, digital method of making and receiving phone calls over the Internet. An SIP trunk acts as a virtual phone line, allowing your application to place and receive calls without relying on legacy phone infrastructure.

Unlike traditional analog or ISDN phone lines, SIP trunks can handle not only voice calls but also multimedia communication. This includes voice, video, and messaging all over a single IP connection. SIP trunking is quickly becoming the industry standard, with many countries actively phasing out older ISDN networks.

In the workflow described in this guide:

• Twilio acts as your telephony provider, receiving calls from the public phone network and routing them using SIP.

• LiveKit receives the SIP call, bridges it into a real-time audio room, and enables your Voice AI agent to participate in the conversation.

Alternatives to SIP: H.323, WebRTC (Web Real-Time Communication), XMPP (Extensible Messaging and Presence Protocol), MGCP (Media Gateway Control Protocol), Proprietary Protocols.

3.Behind the Ring: The Inbound Call Workflow

Let’s break down the typical workflow for routing inbound calls to a Voice AI agent:

• A customer dials your Twilio phone number.

• Twilio receives the call and routes it through an Elastic SIP Trunk based on your SIP configuration.

• Twilio forwards the call via SIP (Session Initiation Protocol) to LiveKit’s SIP endpoint.

• LiveKit authenticates the SIP trunk credentials and searches for a matching dispatch rule.

• LiveKit creates a SIP participant for the caller and places the caller into a LiveKit room based on the dispatch rule.

• The Voice AI agent answers the call, processes the caller’s speech, and responds conversationally.

4. Let’s dive into practical implementation

Follow the GitHub repo for a smoother implementation.

Before your Voice AI agent can start handling calls, you’ll need to configure the essential components that connect Twilio and LiveKit. Here’s a step-by-step guide to set up the pipeline and get your real-time voice integration up and running.

4.1 Set Up Twilio Account

• Create a Twilio Account: Sign up for a Twilio account if you haven’t already.

Purchase a Phone Number: In the Twilio Console, buy a phone number that will receive inbound calls.

4.2 Set Up LiveKit Account

• Create a LiveKit Account and Project: Register for LiveKit and create a new project to manage your real-time audio interactions.

• Get Project URL and SIP URI: In your LiveKit project settings, find the Project URL and SIP URI parameters. These are necessary for SIP trunk configuration.
  
  

4.3 Install Twilio CLI

To install with Advanced Package Tool (apt), run the following commands in your terminal:

  wget -qO- https://twilio-cli-prod.s3.amazonaws.com/twilio_pub.asc \
    | sudo apt-key add -
  sudo touch /etc/apt/sources.list.d/twilio.list
  echo 'deb https://twilio-cli-prod.s3.amazonaws.com/apt/ /' \
    | sudo tee /etc/apt/sources.list.d/twilio.list
  sudo apt update
  sudo apt install -y twilio

4.4 Create a SIP trunk

Using the TWILIO UI

• Search Elastic SIP OR Select Elastic SIP Trunking >> Manage >> Trunks.

• Create a SIP trunk.

Using Twilio CLI

Make sure the domain name of your SIP trunk ends with [pstn.twilio.com]

twilio api trunking v1 trunks create \
  --friendly-name "My test trunk" \
  --domain-name "my-test-trunk.pstn.twilio.com"

After running the above command, you will receive TWILIO-TRUNK-ID. Copy your TWILIO-TRUNK-ID and save it

4.5 Configure Twilio trunk for inbound calls

Using the TWILIO UI

• Navigate to Voice >> Manage >> Origination connection policy.

• Select the created origination policy.

• Add the SIP-URI, weight, and priority.

Using Twilio CLI

twilio api trunking v1 trunks origination-urls create \
    --trunk-sid <TWILIO-TRUNK-ID> \
    --friendly-name "LiveKit SIP URI" \
    --sip-url <Livekit-SIP-URI> \
    --weight 1 --priority 1 --enabled