How to embed Twilio Voice in a Retool app for outbound calling

October 17, 2022
Written by
Evan Weiss
Reviewed by

Embed Twilio Voice in a Retool app for outbound calling

Customer-facing teams often find themselves in the unenviable position of needing to manage customer data and customer communications in separate tools. Twilio Voice makes it easy to add telephony to your applications using SDKs and APIs. Twilio Voice is frequently used to add telephony to consumer-facing applications, and why should internal applications be any different?

In this tutorial, you’ll use the Twilio Voice JS SDK to add outbound voice calling capabilities to a lead management application built in Retool. Retool is a development platform for internal tools. Together, Twilio Voice and Retool provide a powerful and easy-to-use toolkit to build custom voice-enabled internal applications.

Retool application for managing leads. The application lists leads in a table on the left. On the right, there"s an input with a phone number entered with a red button saying "End" next to it. Underneath the phone number is a log of the call statuses.


In order to complete this tutorial, you will need the following:

Twilio Setup

Before you can start building in Retool, you’ll need to do some pre-work in the Twilio Console.

1. Create a TwiML App

A TwiML Application tells Twilio how to handle the calls you'll make from your Retool app. Create a TwiML Application following these steps. Specify a name for your TwiML App and note the TwiML App SID for later. Leave all other settings blank, you'll fill them in later on!

2. Create an API Key

Create an API Key and note the key's SID and Secret for later. The API Key will authenticate and authorize you to interact with Twilio APIs.

3. Create a Twilio Service

A Twilio Service is a container for the serverless functions which makes it possible for your Retool app to get a voice access token, and for outbound calls to be routed correctly. Create a Twilio Service following these steps.

4. Set Twilio Service environment variables

In your new Twilio Service, set the following environment variables:

TWIML_APPLICATION_SIDThe TwiML App SID you noted in step one
API_KEY_SIDThe API Key SID you noted in step two
API_KEY_SECRETThe API Key Secret you noted in step two
CALLER_IDThe phone number you obtained when you set up your Twilio account. Make sure to include a + and the country code at the beginning!

Learn how to set environment variables for your Twilio Service here.

5. Configure Twilio Service dependencies

The serverless functions running in Twilio need access to a few software libraries to function properly. Import the following NPM modules:


Learn how to import NPM modules and manage node.js dependencies in your Twilio Service here.

6. Create a Twilio Function to issue voice tokens

In order for the embedded Twilio dialer to authenticate with Twilio's services, it needs a valid access token. To create a function for issuing voice tokens:

  1. Add a function named /voice-token.
  2. Set the visibility of the function to Public.
  3. Copy and paste the contents of voice-token.js into the function and save.
  4. Copy the URL for this function for later use.

Heads up! Setting the visibility of your function to public means that anyone who can guess the URL of your function can issue valid access tokens. You would need to implement authentication on this endpoint for a production use-case!

The Twilio Functions Console displaying the JavaScript contents of the /voice-token function. The left panel displays a list of serverless functions, and /voice-token is selected. The right panel displays an excerpt of the JavaScript function, which retrieves the necessary environment variables and uses them to generate an accessToken.

7. Create a function to route outbound calls

In order for Twilio to know what to do when it receives a call from your app, you need a call routing app. To create a call routing function:

  1. Add another function and name it /twiml-app.
  2. Set the function visibility to Protected.
  3. Copy and paste the contents of twiml-app.js into the function and save.
  4. Copy the URL for this function for later use.

8. Deploy your Twilio Service

Publish your Twilio Functions by clicking Deploy All. This finalizes all the changes you've made.

9. Finish configuring your TwiML App

Remember that empty TwiML App we created in step one? Edit the app settings and provide the full URL for your /twiml-app function as the value of the Voice Configuration Request URL. Specify a Request Method of HTTP POST and save your changes.

This step connects inbound requests from your Retool app to the call routing function.

Retool setup

Now that your Twilio account is set up, let's set up the Retool app.

1. Create a Retool REST API Resource to retrieve voice tokens

In order for your Retool app to authenticate with Twilio, you'll need to get a valid voice token. You’ll configure a REST API resource in your Retool project to query the voice token Twilio Function from your app.

To set this up:

  1. Navigate to the /resources page in your Retool instance and click "Create new > Resource."
  2. Select REST API as the resource type.
  3. Name the resource “Twilio Voice Token”.
  4. Specify the Base URL of your /voice-token Twilio Function as noted in step six of the Twilio setup.
  5. Specify a header with a key of identity and a value of {{}}. This is used to pass the email address of the Retool user to Twilio for use in the token identity.
  6. Save your changes.

2. Import the Retool app code

First, download twilio-dialer-retool-app.json from the Retool app exchange. To import the app code, navigate to the Retool main page and click Create new and select From JSON. Upload the JSON file containing the app code, and name the app.

You may need to connect the getVoiceToken Resource Query to the REST API Resource you created previously. To do this, edit the Retool app and:

  1. Open the bottom panel to view the getVoiceToken Resource Query.
  2. Pick “Twilio Voice Token” from the Resource dropdown.
  3. Save the Resource Query.

3. Explore the Retool app

If everything is working as expected, you should now be able to place outbound calls from the Retool app! The app is set up with a hard-coded table of "leads" to simulate a simple call center app. You can edit the contents of the leads table directly, or simply type your phone number in the field to try calling yourself. Click the Preview button on the top right to try out the application.

4. Learn about how the custom component works

The Twilio Dialer component has been built as a Retool custom component. You can view the code used in the Twilio Dialer component in Retool by clicking on the twilioDevice component and inspecting the IFrame Code field. You can also find the same code at retool-custom-component.html for easy reading!

The Retool builder displaying the JavaScript contents of the Twilio Dialer custom component.  The JavaScript excerpt shows the window.Retool.subscribe function being used to initialize a new Twilio Device.

Also, take 18 minutes to check out our video walkthrough of how this app is put together!

Wrap up

We hope you enjoyed this introduction to Retool, and have a better understanding of how to use Retool Custom Components to extend the functionality of your internal tools. To learn more, check out Retool University!

Evan Weiss is a Sales Engineer at Retool, where he enjoys teaching customers about how to build custom internal software. You can find him at evan[at], as well as on LinkedIn and Twitter.