Is OTP free in Firebase?

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

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

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.