Short answer
Move the project to the Blaze plan in the Firebase console and set a budget alert first. A budget alert only notifies you; it doesn’t cap the bill. For development and app review, add fictional phone numbers with fixed codes, which send no SMS. Then retry the sign-in.
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Why phone sign-in needs billing, and the fix
01
The project is on the free Spark plan
“Verification code SMS messages Pay as you go (Blaze) plan only.” Firebase, Firebase Authentication limits
SMS codes cost money to send, so Firebase only sends them on Blaze. Upgrading adds pay as you go billing. SMS is charged per message, so set a budget alert before real users sign in.
02
You only need it for testing
“Test phone number authentication without sending an actual SMS message.” Firebase, Phone authentication on the web
Fictional numbers set in the console return a fixed code and send no SMS, so they work for development. Anyone with the number and code can sign in, so use hard-to-guess codes, give one to app reviewers only for the review, and change it afterwards. You can add up to 10.
03
After upgrading, the daily limit applies
“Firebase Authentication: 3000 sent SMS/day limit” Firebase, Firebase Authentication limits
Firebase Authentication on Blaze sends up to 3,000 verification SMS a day. For more, upgrade to Identity Platform.
04
Bots can run up the SMS bill
Phone sign-in on Blaze is a target for SMS pumping, where bots request codes to numbers that earn them money. Consider App Check and limiting SMS to the countries you serve, and watch the SMS count in the console.
Checked against the vendor’s docs on : Firebase docs: Authentication limits; Firebase docs: Phone authentication on the web.
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Related: Firebase auth/too-many-requests · Firebase phone auth invalid app credential · Firebase auth/invalid-phone-number · Firebase missing initial state