Android Phone OTP Signup Module

I’m finishing up an Android-only app and now need a clean, reliable phone-number sign-up flow. The user experience I’m after is straightforward: the user enters their mobile number, an SMS with a one-time code arrives, they type that code, and the account is instantly created and authenticated—no emails, no social logins, just a quick OTP.

You are free to use Firebase Authentication, Twilio Verify, or any comparable SMS gateway as long as the integration is secure, respects rate limits, and gracefully handles edge cases such as delayed or expired codes. I have a Firebase project ready, but if you prefer another service just let me know so we can align API keys and pricing before you start.

Deliverables
• Android Studio source (Kotlin or Java) with the sign-up screen, OTP entry screen, and success callback
• Backend or cloud function (if required) that issues and validates the OTP
• Simple persistence of the logged-in user (SharedPreferences or Room) so the session survives app restarts
• A short README explaining how to rebuild, swap SMS providers, and move the module into the main app

I’ll test with several phone numbers and expect the entire round-trip—from number entry to verified login—to complete in under 10 seconds on a typical 4G connection. If that sounds doable, I’m ready to get started right away.