Is OTP free in Firebase?

From the use of firebase OTP verification using a phone number is FREE .

In Firebase’s case you will probably be on the free tier (as long as you are not guzzling data) up to 100k users . Think about this for a moment. You will have 100k users on your app. Even if you only add advertising you should be able to make the $25 a month to cover the Firebase FLAME costs.

Auth is free . Firebase Authentication service is free in all plans.