Voice and SMS One-Time Passwords with Python and Twilio

May 25, 2020
Written by
Dotun Jolaoso
Opinions expressed by Twilio contributors are their own

Voice and SMS One-Time Passwords with Python and Twilio

One-time passwords (OTPs) contain numeric or alphanumeric codes that are used to provide an extra layer of security for your applications, by ensuring that a user is authenticated for a particular transaction or a login session. In this tutorial, we’ll be building a simple Flask application that generates and validates OTPs that are delivered to users via Voice or SMS channels using Twilio.

Technical requirements

To follow along, you’ll need the following:

  • A free Twilio Account. If you use this link to register, you will receive $10 credit when you upgrade to a paid account.
  • Python 3.6 or newer.
  • A phone line that can receive voice calls and/or SMS.

Creating a Python environment

Let’s create a directory where our project will reside. From the terminal, run the following command:

$ mkdir twilio_otp

Next, cd into the project directory and run the following command to create a virtual environment.

$ python -m venv venv

To activate the virtual environment on a Linux or MacOS computer, run the following command:

$ source venv/bin/activate

If you are using a Windows computer, then the activation command is different:

$ venv\Scripts\activate

Next, we’ll need to install all the dependencies our project will need:

  • Flask: a Python web framework.
  • Twilio: A helper library that makes it easy to interact with the Twilio API.
  • Requests: A library for making HTTP requests.
  • Python-dotenv: A library for importing environment variables from a .env file.

To install all the dependencies at once, run the following command:

$ pip install flask twilio requests python-dotenv

Creating the Views

Create a main.py file at the root of the project’s directory and add the following code to the file:

from flask import Flask, request, render_template

app = Flask(__name__)

@app.route('/generate', methods=['GET', 'POST'])
def generate():
    if request.method == 'GET':
        return render_template('generate.html')

@app.route('/validate', methods=['GET', 'POST'])
def validate():
    if request.method == 'GET':
        return render_template('validate.html')

if __name__ == '__main__':

Here, we’ve defined two routes, generate and validate. These two routes will render generate and validate templates respectively that we shall be creating shortly. Run the following command from the terminal to start the web server:

$ python main.py

To stop the server press Ctrl-C.

Creating the Base Layout

Our application is going to have two templates, one for allowing users to input their phone numbers to receive the OTP code and the other for validating an OTP code. However, before we create those templates, we’ll be creating a base template so that we can provide support for template inheritance and avoid duplicating code.

As it is consistent for Flask applications, our templates will reside in a templates sub-directory. Run the following command from your terminal to create the directory:

$ mkdir templates

Within the templates directory, create a base.html file and add the following code to the file:

<!doctype html>

    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">

    <title>Generate OTP</title>

    <!-- Fonts -->
    <link href="https://fonts.googleapis.com/css?family=Nunito:200,600" rel="stylesheet">

    <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.5.0/css/bootstrap.min.css">

    <!-- Styles -->
        body {
            background-color: #fff;
            color: #636b6f;
            font-family: 'Nunito', sans-serif;
            font-weight: 200;
            height: 100vh;
            margin: 0;

        .container {
            max-width: 600px;
            margin: 100px auto;

    <div class="container">
        {% with messages = get_flashed_messages(with_categories=true) %}
        {% if messages %}
        {%- for category,  msg in messages %}
        <div class="alert alert-{{ category }} alert-dismissible">
            <a href="#" class="close" data-dismiss="alert" aria-label="close">&times;</a>
            <strong>{{ msg }}</strong>
        {% endfor -%}
        {% endif %}
        {% endwith %}

        {% block body %} {% endblock %}
        <!-- jQuery library -->
        <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

        <!-- Latest compiled JavaScript -->
        <script src="https://stackpath.bootstrapcdn.com/bootstrap/4.5.0/js/bootstrap.min.js"></script>


Here we’ve defined an HTML5 layout and included Bootstrap and jQuery. The layout uses Jinja2 blocks which serve as a placeholder for giving derived templates the ability to insert their own content. The layout also includes an alert component for displaying notifications based on Flask’s message flashing feature.

Creating the Template For Generating OTP

Within the templates directory, create a generate.html file and add the following code to the file:

{% extends "base.html" %}

{% block body %}
<h1> Generate OTP </h1>
<form method="post" action="">
    <div class="form-group">
        <input type="tel" class="form-control" name="phone_number" placeholder="Enter your phone number">
    <div class="form-group">
        <div class="form-check form-check-inline">
            <input class="form-check-input" type="radio" name="channel" value="voice" checked>
            <label class="form-check-label">Voice</label>
        <div class="form-check form-check-inline">
            <input class="form-check-input" type="radio" name="channel" value="sms">
            <label class="form-check-label">SMS</label>
    <button type="submit" class="btn btn-lg btn-info">Request For OTP</button>
{% endblock %}

We’ve created a HTML file that extends the base.html layout file we created earlier. The file contains a HTML form for allowing the user to input their phone number as well as select the preferred channel by which they would like to receive the OTP code. The action attribute of the form is set to an empty string. This is done so that the form will be submitted back to the current URL that appears in the address bar.

Start the application as shown in the previous section and then enter http://localhost:5000/generate in your address bar to see how this template looks.

generate otp template

Creating the Template For Validating OTP

Create a validate.html file within the templates directory and add the following code to the file:

{% extends "base.html" %}

{% block body %}
<h1> Validate OTP </h1>
<form method="post" action="">
    <div class="form-group">
        <input type="text" class="form-control" name="otp_code" placeholder="Enter the OTP code here" required>
    <button type="submit" class="btn btn-lg btn-info">Validate OTP</button>
{% endblock %}

This HTML file also extends the base.html layout and adds a form that allows the user input the OTP code that was received so that it can be validated. The action attribute of the form is also set to an empty string.

Enter http://localhost:5000/validate in your browser’s address bar to see this template.

validate otp template

Setting up Twilio

After you sign up for an account on Twilio, head over to your Twilio Console and set up a Twilio phone number that can make voice calls as well as send SMS messages. This is the number from which the OTPs are going to be sent. You can add a phone number to your account in the Buy a Number page if you don’t already have one.

If you are using a trial Twilio account you will need to verify your own phone number with Twilio before it can receive calls or SMS messages from the Twilio number. You can do that here.

On your Twilio Console, copy your Account SID and Auth Token. We are going to need these values to authenticate with the Twilio service. At the root of the project’s directory, create a .env file and add paste your Twilio credentials, along with the Twilio phone number associated with your account:


For the phone number use the canonical E.164 format.

twilio account sid and auth token

Generating the OTP

Since we’ve been able to create the views for generating and validating the OTP code, we can add the functionality of generating and validating the code. The responsibility of the actual generation and validation of the OTP code will be delegated to a third-party API called GenerateOTP.

Next, in the main.py, replace the existing import statements and Flask applicaiton instance creation at the top of the file with the following updated code:

import os
from flask import Flask, request, session, flash, redirect, url_for, render_template
from dotenv import load_dotenv
from twilio.rest import Client
import requests

app = Flask(__name__)
app.secret_key = 'secret'
twilio_client = Client()
generateotp_url = 'https://api.generateotp.com/'

Here, we’ve imported all the major dependencies our project will be needing and sets up our project:

  • The load_dotenv() function loads our environment variables from the .env file.
  • The app.secret_key setting is used to sign the user session in Flask, so that information stored within it is secure.
  • The twilio_client object will be used for interacting with the Twilio API.
  • generateotp_url is the base URL for the GenerateOTP API.

Next, edit the generate() function we created earlier with the following code:

@app.route('/generate', methods=['GET', 'POST'])
def generate():
    if request.method == 'GET':
        return render_template('generate.html')
    phone_number = request.form['phone_number']
    channel = request.form['channel']
    error = None
    if not phone_number:
        error = 'Phone Number is required'
    if channel != 'voice' and channel != 'sms':
        error = 'Invalid channel'
    if error is None:
        formatted_phone_number = phone_number[1:]
        session['phone_number'] = formatted_phone_number
        otp_code = make_otp_request(formatted_phone_number)
        if otp_code:
            send_otp_code(phone_number, otp_code, channel)
            flash('OTP has been generated successfully', 'success')
            return redirect(url_for('validate'))
        error = 'Something went wrong, could not generate OTP'
    flash(error, 'danger')
    return redirect(url_for('generate'))

Add the following auxiliary functions also in the main.py file.

def make_otp_request(phone_number):
    r = requests.post(f"{generateotp_url}/generate",
                      data={'initiator_id': phone_number})
    if r.status_code == 201:
        data = r.json()
        otp_code = str(data["code"])
        return otp_code

def send_otp_code(phone_number, otp_code, channel):
    if channel == 'voice':
        return send_otp_via_voice_call(phone_number, otp_code)
    if channel == 'sms':
        return send_otp_via_sms(phone_number, otp_code)

def send_otp_via_voice_call(number, code):
    outline_code = split_code(code)
    call = twilio_client.calls.create(
        twiml=f"<Response><Say voice='alice'>Your one-time password is {outline_code}</Say><Pause length='1'/><Say>Your one-time password is {outline_code}</Say><Pause length='1'/><Say>Goodbye</Say></Response>",

def send_otp_via_sms(number, code):
    messages = twilio_client.messages.create(to=f"{number}", from_=os.getenv(
        'TWILIO_NUMBER'), body=f"Your one-time password is {code}")

def split_code(code):
   return " ".join(code)

In the generate() function, we now carry out the necessary checks to ensure the phone number and the channel fields are present in the request when the form is submitted. It’s always a best practice to do server side validation even if validation is still carried out by the client.

To generate the OTP code, the GenerateOTP API needs an initiator_id which serves as an identifier for the particular user the OTP belongs to. In our case, we can use the phone number as the initiator_id. We expect the phone number in the incoming request to be in the canonical E.164 format, so we have to remove the leading plus “+” sign as the GenerateOTP doesn’t accept it as a valid character in the initiator_id field. The formatted phone number is then stored in the Flask user session, so that it can be recalled later when the OTP is validated.

A call is then made to the make_otp_request() function.  This function handles making a HTTP POST request to the GenerateOTP API to generate the OTP code.

Once the OTP code has been successfully generated, a call is made to the send_otp_code() function which either routes the notification through a voice call or SMS depending on the channel that was specified by the user in the submitted form.

In the send_otp_via_voice_call() function, the OTP code is passed as a string to the split_code() function. The split_code() function takes the OTP code and adds spaces between each digit in the code so that Twilio pronounces each number instead of pronouncing the whole word. You can read more about this trick here.

Then a call is placed to the provided phone number. The argument is a predefined message format and set of instructions to speak the code in the Twilio Markup Language (TwiML), which at its core is an XML document with special tags defined by Twilio. The send_otp_via_sms() function sends the OTP code via SMS.

Barring there are no validation or other issues, a message is flashed to the user with a success category and then a redirect is made to the validate endpoint. If there are errors, a message is also flashed to the session but with a danger category and a redirect back to the generate endpoint to give the user a chance to try again.

Validating the OTP

Now that we’ve handled the logic for generating the OTP code, let’s handle the corresponding logic for validating the OTP code. In the main.py file, edit the validate() function we created earlier with the following code:

@app.route('/validate', methods=['GET', 'POST'])
def validate():
    if request.method == 'GET':
        return render_template('validate.html')
    otp_code = request.form['otp_code']
    error = None
    if not otp_code:
        error = 'OTP code is required'
    if 'phone_number' in session:
        phone_number = session['phone_number']
        error = 'Please request a new OTP'
    if error is None:
        phone_number = session.get('phone_number')
        status, message = verify_otp_code(otp_code, phone_number)
        if status is True:
            flash(message, 'success')
            del session['phone_number']
            return redirect(url_for('validate'))
        elif status is False:
            flash(message, 'danger')
            return redirect(url_for('validate'))
        error = 'Something went wrong, could not validate OTP'
    flash(error, 'danger')
    return redirect(url_for('generate'))

Add the following auxiliary function also in main.py:

def verify_otp_code(otp_code, phone_number):
    r = requests.post(f"{generateotp_url}/validate/{otp_code}/{phone_number}")
    if r.status_code == 200:
        data = r.json()
        status = data["status"]
        message = data["message"]
        return status, message
    return None, None

Here, similarly to what we did with the generate() function, a validation check is carried out to ensure the otp_code is present in the form submitted with the request. The phone number of the user is retrieved from the user session and then a call is made to the GenerateOTP API to validate the OTP code that was received. Depending on the status of the validation, the appropriate response is sent back to the user.


From the terminal, run the following command to start the application

$ python main.py

Navigate to the http://localhost:5000/generate URL to see the OTP request page.

otp request page

Enter your phone number in the canonical E.164 format. For example, for a number in the United States use for format +1AAABBBCCCC, where +1 is the country code, AAA is the area code and BBB-CCCC is the local phone number. Specify the channel by which you would like to receive the OTP and click the “Request For OTP” button. Once you receive the OTP, you can go ahead and enter it in the validate page to see if it is valid or not.

otp validate page


In this tutorial we’ve learned how to build a simple Flask application for generating, validating as well as delivering OTP codes via voice call or SMS. The source code for this tutorial can be found here on Github.

If your needs are more complex, consider using the Twilio Verify service, which provides a robust user verification system that works over SMS, phone and email. Learn how to implement Twilio Verify in a Flask application.

Dotun Jolaoso

Website: https://dotunj.dev/
Github: https://github.com/Dotunj
Twitter: https://twitter.com/Dotunj_