I’m building an Android-only application and need a clean, secure login flow that relies on one-time passwords sent strictly by SMS. Users will enter their phone number, receive the code, and gain access after successful verification—no usernames or emails involved. You’ll be responsible for wiring up the full authentication cycle, choosing a reliable SMS gateway, and writing the Android code (Kotlin or Java—your call) that: • accepts the phone number, • triggers the SMS OTP dispatch, • validates the returned code with sensible expiry and limited retries, • handles resend logic, and • stores / refreshes session tokens once the user is verified. Deliverables must include the documented source code, any backend or cloud function needed to send and verify the OTP, and brief setup instructions so I can run it in my environment without guesswork. If you have questions about the SMS provider or need sandbox credentials, let me know early so we can keep integration smooth and straightforward.