I've completed implementing Firebase Auth with phone verification . It has no problems until this error shows up.
After call FirebaseAuth.instance.verifyPhoneNumber method, verificationFailed callback is called. FirebaseException says: '[firebase_auth/unknown] An internal error has occurred. [ BILLING_NOT_ENABLED ]'.
I've checked phone authentification sms usage (which is 0). And I turned my GCP api key (android / ios)to no restriction.
It's not working on both ios and android.
Android log:... [firebase_auth/unknown] An internal error has occurred. [ BILLING_NOT_ENABLED ]
I use Spark plan and SMS Usage is currently 0
If you are on the Blaze plan, you will not see this error.
However, on the Spark plan, you should still get 10 free sms messages per day according to their pricing page.
Back when I was in Spark, I had a similar issue (with Firebase Cloud Messaging if I remember correctly) - Go to your Google Cloud Platform and make payment profile for your Firebase project. After that you should not see this error anymore. You can still be on the Spark plan in your Firebase project.
It isn't quite a solution, but it worked for me. You should consider raising an issue with Firebase