When using the firebase/quickstart-ios - specifically the part on authentication - phone number authentication, I ran into an error (internal error) which I had to debug (https://github.com/firebase/quickstart-ios/issues/1619). I was getting a BILLING_NOT_ENABLED error. To solve it, which took me some time, I enabled a billing account for my Firebase project on https://console.cloud.google.com/billing/projects