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This guide walks you through connecting incoming Twilio calls to an Ultravox agent.

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Prerequisites

  • Node.js 20 or higher
  • A Twilio account with:
    • Account SID
    • Auth Token
    • Phone Number
  • An Ultravox API key
  • For incoming calls: A publicly accessible URL for your webhook (e.g., using ngrok)

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Set-up and Installation

1

Set Up Your Project

Create a new directory for your project and initialize it:
mkdir ultravox-incoming-calls
cd ultravox-incoming-calls
npm init -y
2

Install Dependencies

npm install express twilio
3

Create Your Server

Create a new file called server.js with the following code. This code can also be found on GitHub.Note: Make sure to add your Ultravox API key.
import express from 'express';
import https from 'https';
import twilio from 'twilio';

const app = express();
const port = 3000;

// Configuration
const ULTRAVOX_API_KEY = 'your_ultravox_api_key_here';
const ULTRAVOX_API_URL = 'https://api.ultravox.ai/api/calls';

// Ultravox configuration
const SYSTEM_PROMPT = 'Your name is Steve. You are receiving a phone call. Ask them their name and see how they are doing.';

const ULTRAVOX_CALL_CONFIG = {
    systemPrompt: SYSTEM_PROMPT,
    model: 'fixie-ai/ultravox',
    voice: 'Mark',
    temperature: 0.3,
    firstSpeaker: 'FIRST_SPEAKER_AGENT',
    medium: { "twilio": {} }
};

// Create Ultravox call and get join URL
async function createUltravoxCall() {
    const request = https.request(ULTRAVOX_API_URL, {
        method: 'POST',
        headers: {
            'Content-Type': 'application/json',
            'X-API-Key': ULTRAVOX_API_KEY
        }
    });

    return new Promise((resolve, reject) => {
        let data = '';

        request.on('response', (response) => {
            response.on('data', chunk => data += chunk);
            response.on('end', () => resolve(JSON.parse(data)));
        });

        request.on('error', reject);
        request.write(JSON.stringify(ULTRAVOX_CALL_CONFIG));
        request.end();
    });
}

// Handle incoming calls
app.post('/incoming', async (req, res) => {
    try {
        console.log('Incoming call received');
        const response = await createUltravoxCall();
        const twiml = new twilio.twiml.VoiceResponse();
        const connect = twiml.connect();
        connect.stream({
            url: response.joinUrl,
            name: 'ultravox'
        });

        const twimlString = twiml.toString();
        res.type('text/xml');
        res.send(twimlString);

    } catch (error) {
        console.error('Error handling incoming call:', error);
        const twiml = new twilio.twiml.VoiceResponse();
        twiml.say('Sorry, there was an error connecting your call.');
        res.type('text/xml');
        res.send(twiml.toString());
    }
});

// Start server
app.listen(port, () => {
    console.log(`Server running on port ${port}`);
});

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Configure Twilio Webhook

1

Make Your Server Publicly Accessible

Use ngrok or a similar service to create a public URL for your local server:
ngrok http 3000
2

Set Up Webhook

  1. Go to your Twilio Console
  2. Navigate to your phone number’s configuration
  3. Under “Voice & Fax”, set the webhook URL for incoming calls to: https://your-ngrok-url/incoming

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Start the Server

Run the server: 
node server.js
Now, when someone calls your Twilio number, they’ll be connected to your AI assistant.

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Next Steps

Ultravox Realtime provides telephony integrations for Telnyx, Twilio, and Plivo. Learn more here.

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Additional Resources

  • Twilio Documentation
  • Express.js Documentation
  • ngrok Documentation