Firebase provides a limit of 10K/Month phone user authentication for free. So I thought it is a good way to implement OTP system. I compiled the example app of FirebaseUI and I found a "verification using phone" button on the app. It was initially showing "unknown error occured" when trying to test it with my phone number. I looked on the internet and it said to enable Phone authentication method from the Firebase Project's settings page. I enabled it and there was an option in the Firebase Project page where one can add phone numbers for testing. I added my number and tested the app again and it worked. But my aim is to verify any given number with an OTP, not just the number saved for testing. Can anyone provide me an example file of how to do it?