One of the best free one-time password (OTP) sender APIs is provided by Twilio. Twilio offers a free tier that allows you to send a limited number of OTPs per month. You can sign up for a free account to get started.

Twilio Free OTP API

• Website: Twilio
• Features: Secure delivery of OTPs, supports SMS, voice, and email channels, highly reliable.
• Free Tier: Includes a limited number of free OTPs per month.

To use Twilio's OTP API, you need to:

1. Sign up for a free Twilio account.
2. Verify your phone number.
3. Get your Account SID and Auth Token.
4. Use Twilio's API to send OTPs.

Example code snippet to send an OTP using Twilio (Python):

python
from twilio.rest import Client

# Your Account SID and Auth Token from twilio.com/console
account_sid = 'your_account_sid'
auth_token = 'your_auth_token'
client = Client(account_sid, auth_token)

message = client.messages.create(
    body='Your OTP code is 123456',
    from_=' 1234567890',
    to=' 0987654321'
)

print(message.sid)  


Note: Remember to replace 'your_account_sid', 'your_auth_token', ' 1234567890', and ' 0987654321' with your actual Twilio account SID, Auth Token, Twilio phone number, and recipient phone number respectively.